Radiator Caps, Thermostats, and Overheating Info - BMW Luxury Touring Community
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post #1 of 33 Old May 23rd, 2015, 2:09 pm Thread Starter
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Radiator Caps, Thermostats, and Overheating Info

I know there have been a few discussions about overheating and weather the problem is the radiator cap, thermostat, coolant, air in the system, etc. None seemed to offer more than guesses. I recently had a recurring overheat condition that I needed to resolve and wanted to share the results of my investigation.

My bike would overheat while in extended traffic, or moving when slowly (stop and go) with two up. The needle (which recently seemed to hover a little on the low side of the mid point when moving fast) would steadily climb to the red zone followed by the light.

Bike:
2002 K1200LT with 52K miles. Mine since new and the cooling system has always been serviced by a dealer every two years.

Diagnostics:
-Checked that both fans were kicking on fully - Good
-Checked the overfill tank to ensure there was fluid present between full and min - Good
-Checked that the radiators were not covered in bugs or trash - Good
-Fluid was changed by the dealer a year previously - Good
-Took temperature of both lower hoses (each side of the thermostat) and the temps were 20 degrees cooler on the left side (as viewed from the riders sitting position) than the right side (fluid slowed by thermostat but not completely blocked) - Good I think?
-Check to ensure fluid in the radiators was all the way up to the cap (you have to remove the left side plastic to get to the cap)- Good

Plan:
Step 1. Buy parts
Thermostat 17.11.1.464.985, $44.91
O-Ring 17.11.1.464.984, $5.59
Thermostat plastic cover 17.11.1.464.879, $6.65
Fixing clip 17.11.1.464.991, $3.83
Radiator cap 17.11.7.675.523, $33.58 (This is a new part number from the original 17.11.1.464.983 on my bike.)
All parts bought from Morton's BMW on-line parts ordering system.

Step 2. Wait for parts and make a plan. Plan was to change all of these parts and hopefully find the culprit. From my automotive experience this seemed like a thermostat that wasn't opening all the way or was stick in a partially open position. Turns out this was a good reminder to not assume one thing is like another and to work step by step.

Step 3. Garage time. Starting with a bike that had been sitting overnight (you never want to do cooling system work on a hot engine), I removed the plastic on both sides of the bike and the lower center section so that would have access to the radiators, cap, and thermostat. This proved to be adequate.

Before I started working on the system I decided to use compressed air to blow out the radiators back to front to ensure there wasn't a bunch of junk stuck between the fins. I couldn't see anything with a flashlight but wanted to be thorough. With the plastic off now was the perfect time. I carefully ran my air nozzle between the fan blades without touching the radiator fins over all areas of the back of the radiators that I could access (do not let compressed air spin the fans, this is not good for them and can cause a failure by spinning the too fast or in the wrong direction) . Ok, special note here, wear your safety goggles. I observed a shower of small grit, sand, bug legs, and dust coming out of the front of the radiators. Not enough to cause my overheat I think but a good thing to do and I think I'll add this to my regular maintenance routine, at least yearly.

Next I loosened the radiator cap (confirmed full condition up to the neck). I placed a clean pan under the front of the bike below the junction of the two lower radiator hoses. The thermostat is housed in the plastic cover and is connected on one side by a standard radiator clamp and the other side by the U-shaped fixing clip. Using a flat blade screwdriver and pliers I carefully removed the fixing clip. To my surprise the thermostat housing did not simply pop off of the bottom of the right radiator. I then released the screw clamp and pulled the right hose to drain the fluid.

Once the fluid was out I used a combination of pliers, screwdrivers, cussing, praying, time and finally a heat gun applied to the junction to expand the plastic a bit to get the old thermostat cover off without breaking anything. I did chew up the old cover a bit but was not concerned as I was patting myself on the back for purchasing a new one in advance. The heat gun is really the ticket to getting the old cover out.

With the cover off I was able to get to the thermostat. Everything looked good and it appeared that it was in the closed position. No excess crud in the lower hoses either. The thermostat on this bike has a large spring that is compressed to open the flow of hot coolant or is expanded fully to block the flow in cold conditions. The spring is not the control. In the center of the thermostat is a capsule that contains what I'll call a piston. When the coolant is cold, the spring pressure is able to force the piston back into the capsule and close completely. When the coolant is hot, the piston is forced out of the capsule and overcomes spring pressure to let coolant flow. In the cold position the piston sticks out about 1 cm. In the hot position the piston extends just short of 2 cm. I confirmed all of this by doing a hot cold test with both the new and old thermostat. Both worked identically, fully extending starting at 180 degrees F and complete by 220 degrees F. However, it is important to note that the piston will not simply shrink back into the capsule. It requires the full pressure of the spring to push it back into the capsule when cold. Also confirmed on both thermostats and very different from an automobile thermostat.

I replaced the thermostat with the new one, cover, o-ring, and clip. It is a bit tricky to get everything aligned and back in place. First ensure the thermostat piston is in the fully cold position if you have been testing it like I did. The thermostat cover has a tab that aligns it to the radiator neck to ensure everything is in the right position. The manual notes that you should chill the cover in the freezer to make installation easier. I did that and also found it helpful to heat the lower radiator neck at the same time and to wet the new o-ring in the pan of antifreeze. This is a juggling game as the thermostat keeps wanting to slip out of alignment. I also found it possible to stick a finger inside of the back of the housing as I was trying to get the housing into the radiator neck to help jiggle thins into alignment. Once everything is pushed into position carefully work the clip into position and get it fully seated then reattach the lower hose and tighten the clamp.

Next begin refilling the system. I took a large squeeze bottle with a foot long piece of small diameter hose and filled the system through the radiator cap inlet. Took a little while and I continually squeezed the hoses to try and get air out. Once full I capped the system off with the new cap.

After everything is full but before you put all of the plastic back on run the bike around the block a few laps and get it fully warmed up to check if the temp is running as it should. If it seems to be running a little hot, squeeze the hoses (wearing gloves) to move air bubbles to the top and then when it is cool enough remove the radiator cap and top the system again. Also ensure the overflow tank is at the right level.

The old cap looked to be in good shape but since I didn't find a problem with the thermostat I now began to suspect the cap. Of course if I had tested the cap first I could have saved time and money but then I wouldn't know as much as I do now.

The system is now working fine. When the heat does come up, the fans come on the needle does not go into the red zone and soon begins heading back down.

Still didn't know exactly what caused the problem yet but I know that pressure is critical in a cooling system. An increase in pressure, controlled by the radiator cap, raises the boiling point of the fluid in the system. Too little pressure and the system boils over, too much pressure and something might just blow. Took the old radiator cap to the auto parts store today and asked to use the radiator pressure tool set (they loan specialty tools at many auto parts stores, just ask). Got the blank look from the clerk but eventually convinced him that such a thing existed and that I could safely use it right there in the store. Found the kit (had to show him) and located the test pump and adapter for the small radiator cap. Our cap is labeled for a pressure of 1.5 Bar (a bit less than 22 PSI). The cap should hold pressure to just below the marked limit and should release pressure at and above that level (how fluid gets back into the overflow tank). I pumped up the pressure and it went right past 22 PSI to the max of 30 PSI (as high as the pump goes) and held firm...ah ha! My system was overheating because it could not release the pressure. Lucky I didn't blow a hose off.

And the lesson of the day is, start by checking the easy things. Fluid, obstructions, radiator cap pressure. Then go on to worrying about the thermostat.

Photos attached are to show the difference between the new and old radiator cap (new on left). Thermostats in cold position and hot position (new on left), and the fully extended measurement of the piston.

Good luck all!
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Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.

Last edited by silverlt2002; May 23rd, 2015 at 2:26 pm.
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post #2 of 33 Old May 23rd, 2015, 5:02 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

That's quite a write-up. Thanks!

Dave Beck
'16 K1600GTLE
'07 K1200LT (sold 9/24/16)
'74 Suzuki GT750L (long, long gone)


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post #3 of 33 Old May 23rd, 2015, 5:23 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Steve,
Excellent write up.
See the attached coolant flow diagram from BMW repair manual that I enhance a bit with a dotted Red arrow to show the optional Thermostat path. Diagram helps to explain why the thermostat is not very standard compare to a car. The whole double radiators flow is a bit strange...

There has been so many defective caps on K1200 as they get old, that I changed mine last year after 13 years of usage (at 87,000 miles) as a preventive measure.

Every 3 years (on average depending on mileage), I remove both radiators completely (including the fans shroud) and I soaked them in hot water for 24 hours. After this, I use a car wash moderate pressure washer to remove rest of crap that did not came out by soaking - be careful with these delicate fins. I do not even do dirt or gravel roads and you cannot image the crap that I find in the soaking bucket every time I do this.
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John (Montreal, CANADA)
K1200RS (2002 IceBlue/Red - 96,000 miles)
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post #4 of 33 Old May 23rd, 2015, 6:27 pm Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

I must say that I was shocked when I found the cap was not releasing any pressure. I was sure if it was defective it would be releasing at low pressure. Guess this should be a regular maintenance check. I had asked the dealer previously (2 years ago) to replace the cap during a service and they gave me the, "oh those never fail, we don't even keep them in stock".

Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.
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post #5 of 33 Old May 23rd, 2015, 6:27 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

So the left radiator is always flowing and the right
only when the thermostat opens?
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post #6 of 33 Old May 23rd, 2015, 6:51 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

good write up thanks, and as some of us already know and you pointed it out, always check the simplest and easy items first.

Regards Linton
From the land of Kangaroo's and Koalas
and no koalas are not Bears



2002 K1200LT
2010 Suzuki GSX 1250FA (the Wifes)
2004 Cub Kamparoo Sprint
My Toys
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post #7 of 33 Old May 23rd, 2015, 7:48 pm Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

Quote:
Originally Posted by Buff8stuff View Post
So the left radiator is always flowing and the right
only when the thermostat opens?
Took me a little while of squinting and scratching myself to come up with what I think is the answer. Looking at the helpful flow chart posted by Sailor, I think the coolant only flows fully through when the thermostat is open. I know it looks like there is a bypass but I believe the white arrow on the bottom of the right radiator will only flow when the thermostat is open. The dashed red arrow from the engine goes to the bottom of the thermostat (spring and capsule side) and I think that when the temp in that line is hot enough it is what triggers the thermostat to open and allow the full flow of coolant around the rest of the diagram.

Just my guess but it is the only way that those arrows make sense to me.

Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.
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post #8 of 33 Old May 23rd, 2015, 7:52 pm Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

Quote:
Originally Posted by Axle View Post
good write up thanks, and as some of us already know and you pointed it out, always check the simplest and easy items first.
Definitely should have tested the cap first. Then again, without having a known good (brand new) thermostat to do a direct temperature comparison against the old one I would have always wondered if I should have replaced it. Now that I know what to look for in movement of the little piston, I can diagnose one before I buy a replacement in the future. Hoping this saves someone else some money and frustration.

Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.
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post #9 of 33 Old Jun 15th, 2015, 12:51 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

I had exactly the same problem and started searching these forums. Found this thread and voila someone else had the same overheating issues.Bought a new radiator cap, put it on, problem solved.
Thanks to silverlt2002 for the extensive write-up as it saved me a lot of time and money.
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post #10 of 33 Old Jun 16th, 2015, 12:42 pm Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

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Originally Posted by Offside View Post
I had exactly the same problem and started searching these forums. Found this thread and voila someone else had the same overheating issues.Bought a new radiator cap, put it on, problem solved.
Thanks to silverlt2002 for the extensive write-up as it saved me a lot of time and money.
Thanks, that makes the time I spent writing it up worth the effort. Glad to have helped.

Steve

Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.
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post #11 of 33 Old Jun 16th, 2015, 1:04 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Quote:
Originally Posted by silverlt2002 View Post
Took me a little while of squinting and scratching myself to come up with what I think is the answer. Looking at the helpful flow chart posted by Sailor, I think the coolant only flows fully through when the thermostat is open. I know it looks like there is a bypass but I believe the white arrow on the bottom of the right radiator will only flow when the thermostat is open. The dashed red arrow from the engine goes to the bottom of the thermostat (spring and capsule side) and I think that when the temp in that line is hot enough it is what triggers the thermostat to open and allow the full flow of coolant around the rest of the diagram.

Just my guess but it is the only way that those arrows make sense to me.
I would think the suction side of the water pump is from the block and output is the hose off the pump which goes to the thermostat, when stat closed flow would be through the small hose to the block and as the coolant warms up some to total flow would be through the rads.
There has to be flow through the motor especially the head all the time, I once in an over tired state put a head gasket on wrong blocking the water flow, 15 seconds after I started the motor the water was popping in the head, it was boiling. That was on a cast iron head and aluminum transfers heat faster. Had to pull the head off and put a new gasket in the right way.
I checked warm up of my cylinder head with plastic off so I could touch the aluminum and at 30 degrees F it was warm to touch in approx 30 seconds. It had not been run all winter and was cold.

Gary
2018 R1200RT
Past rides
2012 K1600GTL
2000 K1200LT
1992 K1100LT
2000 V Star 650/Velorex sidecar
1985 K100RT
1965 R60/2
1960 AJS 500 single
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post #12 of 33 Old Jun 16th, 2015, 9:23 pm Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

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I would think the suction side of the water pump is from the block and output is the hose off the pump which goes to the thermostat, when stat closed flow would be through the small hose to the block and as the coolant warms up some to total flow would be through the rads.
There has to be flow through the motor especially the head all the time, I once in an over tired state put a head gasket on wrong blocking the water flow, 15 seconds after I started the motor the water was popping in the head, it was boiling. That was on a cast iron head and aluminum transfers heat faster. Had to pull the head off and put a new gasket in the right way.
I checked warm up of my cylinder head with plastic off so I could touch the aluminum and at 30 degrees F it was warm to touch in approx 30 seconds. It had not been run all winter and was cold.
I understand your thinking and you may be right and maybe I'm only a bit wrong. From my observation of the thermostat it does not seem like much if any fluid actually passes through in that direction due to obstruction by the rather thick plastic body of the thermostat. The only way the thermostat gets the temperature signal is by hot or cold fluid at the bottom of the capsule which aligns with that small hose (shown by dashed arrow in the chart). It does not react to temperature from the hoses at the top of the thermostat. Once that capsule is heated or cooled then what I called the piston at the top end of the thermostat pushes out or can be squeezed in by the spring allowing fluid to pass at the top end of the thermostat through the larger hoses. Maybe all that it requires is that small amount of circulation from the pump along the dashed arrow hose to allow limited bypass and to signal the thermostat for temperature change. Definitely a different solution than most car thermostats. Of course I could be completely wrong but I would be very surprised to find much fluid passing down the length of the body of the thermostat, the plastic cage just doesn't leave much room at all.

Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.
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post #13 of 33 Old Jun 16th, 2015, 9:42 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Quote:
Originally Posted by silverlt2002 View Post
I understand your thinking and you may be right and maybe I'm only a bit wrong. From my observation of the thermostat it does not seem like much if any fluid actually passes through in that direction due to obstruction by the rather thick plastic body of the thermostat. The only way the thermostat gets the temperature signal is by hot or cold fluid at the bottom of the capsule which aligns with that small hose (shown by dashed arrow in the chart). It does not react to temperature from the hoses at the top of the thermostat. Once that capsule is heated or cooled then what I called the piston at the top end of the thermostat pushes out or can be squeezed in by the spring allowing fluid to pass at the top end of the thermostat through the larger hoses. Maybe all that it requires is that small amount of circulation from the pump along the dashed arrow hose to allow limited bypass and to signal the thermostat for temperature change. Definitely a different solution than most car thermostats. Of course I could be completely wrong but I would be very surprised to find much fluid passing down the length of the body of the thermostat, the plastic cage just doesn't leave much room at all.
I would not expect much flow going by how fast it warms up, when vw started with the double thermostat to restrict flow through the head you got heat from the heater very fast no matter how cold it gets. The K100 and K1100 I had took a long time to warm up. The change is for pollution control, they want the head to warm up fast. It is a small motor and it has a small impeller in the water pump.
I have worked on pumps for a long time and looking at the impeller and volute the coolant has to flow through the hose to the thermostat housing

You did a real nice write up about the thermostat, I was really curious what it looked like and it sounds like a strange beast would prefer not to have trouble with

Gary
2018 R1200RT
Past rides
2012 K1600GTL
2000 K1200LT
1992 K1100LT
2000 V Star 650/Velorex sidecar
1985 K100RT
1965 R60/2
1960 AJS 500 single
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post #14 of 33 Old Jun 17th, 2015, 11:50 am
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Re: Radiator Caps, Thermostats, and Overheating Info

This thread has me baffled. I can understand how a faulty radiator cap can lead to boiling over if it didn't maintain enough pressure to raise the boiling point past where it would at the running temps of the bike but maybe one of the engineers here can enlighten me to how a cap that doesn't release pressure would cause overheating ( not boil over as I don't recall that being the issue) the indicator would run up into the red, no reported loss of fluid. If the T-Stat is working properly the cap would normally vent at the specified pressure from expansion of the liquid ( along with any air ) and then as it cooled it would draw fluid back in with the contraction of the liquid keeping the system full as long as the reservoir had reserve in it.

What is the process that would cause the temp to climb ( all other components being reported normal) if the cap didn't vent at the specified pressure? I would think the worst that would happen is a hose would burst, not overheating.

Some other process must be at work in this case.
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Sugar Hill, GA
2001 K1200LTI – Champagne (current ride) Lazy Susan
1998 R1100RT – Never should have sold it
1974 Yamaha TX 750 Twin. Omni Phase Balanced


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post #15 of 33 Old Jun 17th, 2015, 5:39 pm Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

In theory I absolutely agree with how you view the problem. That is why I suspected the thermostat when I didn't hear fluid being pushed into the overflow tank when it was overheating. However, I can definitely state that replacing my radiator cap which was not releasing pressure solved my overheat problem. I had expected to find that the cap was releasing pressure lower than the necessary 1.5 bar. I was so surprised I ran the test 4 times and it never released any pressure below the maximum of the tester (30 lbs).

I know that the caps are intended to function in both directions to release excess pressure into the overflow and to pull it back later when the system cools. I could not test to find out if the failed cap was able to pull fluid back when necessary, only that it would not release at all.

I'm sure there is a good explanation but I don't have it. Can anyone else who had a problem with the radiator cap confirm if their cap was releasing low or not releasing pressure?

Steve Green
US Army (Ret)
2002 KLT
Titan Silver

I don't like making plans for the day....because then the word "premeditated" gets thrown around in the courtroom.
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post #16 of 33 Old Jun 17th, 2015, 7:11 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

the radiator cap is set to a pre-determined psi as the water heats up it also builds pressure, as the pressure builds up it release (both heat and water) into the overflow bottle, if the cap does not release then the temp inside the radiator will climb causing over heating of the engine. yes the pressure will increase but the water temp also goes up as well, with a cap that does not release over heating will kill the engine well before a hose lets go.

Regards Linton
From the land of Kangaroo's and Koalas
and no koalas are not Bears



2002 K1200LT
2010 Suzuki GSX 1250FA (the Wifes)
2004 Cub Kamparoo Sprint
My Toys
1976 Datsun 260Z
1989 Nissan 300zx TT
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post #17 of 33 Old Jun 17th, 2015, 8:25 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Quote:
Originally Posted by silverlt2002 View Post
I know there have been a few discussions about overheating and weather the problem is the radiator cap, thermostat, coolant, air in the system, etc. None seemed to offer more than guesses. I recently had a recurring overheat condition that I needed to resolve and wanted to share the results of my investigation.

My bike would overheat while in extended traffic, or moving when slowly (stop and go) with two up. The needle (which recently seemed to hover a little on the low side of the mid point when moving fast) would steadily climb to the red zone followed by the light.

Bike:
2002 K1200LT with 52K miles. Mine since new and the cooling system has always been serviced by a dealer every two years.

Diagnostics:
-Checked that both fans were kicking on fully - Good
-Checked the overfill tank to ensure there was fluid present between full and min - Good
-Checked that the radiators were not covered in bugs or trash - Good
-Fluid was changed by the dealer a year previously - Good
-Took temperature of both lower hoses (each side of the thermostat) and the temps were 20 degrees cooler on the left side (as viewed from the riders sitting position) than the right side (fluid slowed by thermostat but not completely blocked) - Good I think?
-Check to ensure fluid in the radiators was all the way up to the cap (you have to remove the left side plastic to get to the cap)- Good

Plan:
Step 1. Buy parts
Thermostat 17.11.1.464.985, $44.91
O-Ring 17.11.1.464.984, $5.59
Thermostat plastic cover 17.11.1.464.879, $6.65
Fixing clip 17.11.1.464.991, $3.83
Radiator cap 17.11.7.675.523, $33.58 (This is a new part number from the original 17.11.1.464.983 on my bike.)
All parts bought from Morton's BMW on-line parts ordering system.

Step 2. Wait for parts and make a plan. Plan was to change all of these parts and hopefully find the culprit. From my automotive experience this seemed like a thermostat that wasn't opening all the way or was stick in a partially open position. Turns out this was a good reminder to not assume one thing is like another and to work step by step.

Step 3. Garage time. Starting with a bike that had been sitting overnight (you never want to do cooling system work on a hot engine), I removed the plastic on both sides of the bike and the lower center section so that would have access to the radiators, cap, and thermostat. This proved to be adequate.

Before I started working on the system I decided to use compressed air to blow out the radiators back to front to ensure there wasn't a bunch of junk stuck between the fins. I couldn't see anything with a flashlight but wanted to be thorough. With the plastic off now was the perfect time. I carefully ran my air nozzle between the fan blades without touching the radiator fins over all areas of the back of the radiators that I could access (do not let compressed air spin the fans, this is not good for them and can cause a failure by spinning the too fast or in the wrong direction) . Ok, special note here, wear your safety goggles. I observed a shower of small grit, sand, bug legs, and dust coming out of the front of the radiators. Not enough to cause my overheat I think but a good thing to do and I think I'll add this to my regular maintenance routine, at least yearly.

Next I loosened the radiator cap (confirmed full condition up to the neck). I placed a clean pan under the front of the bike below the junction of the two lower radiator hoses. The thermostat is housed in the plastic cover and is connected on one side by a standard radiator clamp and the other side by the U-shaped fixing clip. Using a flat blade screwdriver and pliers I carefully removed the fixing clip. To my surprise the thermostat housing did not simply pop off of the bottom of the right radiator. I then released the screw clamp and pulled the right hose to drain the fluid.

Once the fluid was out I used a combination of pliers, screwdrivers, cussing, praying, time and finally a heat gun applied to the junction to expand the plastic a bit to get the old thermostat cover off without breaking anything. I did chew up the old cover a bit but was not concerned as I was patting myself on the back for purchasing a new one in advance. The heat gun is really the ticket to getting the old cover out.

With the cover off I was able to get to the thermostat. Everything looked good and it appeared that it was in the closed position. No excess crud in the lower hoses either. The thermostat on this bike has a large spring that is compressed to open the flow of hot coolant or is expanded fully to block the flow in cold conditions. The spring is not the control. In the center of the thermostat is a capsule that contains what I'll call a piston. When the coolant is cold, the spring pressure is able to force the piston back into the capsule and close completely. When the coolant is hot, the piston is forced out of the capsule and overcomes spring pressure to let coolant flow. In the cold position the piston sticks out about 1 cm. In the hot position the piston extends just short of 2 cm. I confirmed all of this by doing a hot cold test with both the new and old thermostat. Both worked identically, fully extending starting at 180 degrees F and complete by 220 degrees F. However, it is important to note that the piston will not simply shrink back into the capsule. It requires the full pressure of the spring to push it back into the capsule when cold. Also confirmed on both thermostats and very different from an automobile thermostat.

I replaced the thermostat with the new one, cover, o-ring, and clip. It is a bit tricky to get everything aligned and back in place. First ensure the thermostat piston is in the fully cold position if you have been testing it like I did. The thermostat cover has a tab that aligns it to the radiator neck to ensure everything is in the right position. The manual notes that you should chill the cover in the freezer to make installation easier. I did that and also found it helpful to heat the lower radiator neck at the same time and to wet the new o-ring in the pan of antifreeze. This is a juggling game as the thermostat keeps wanting to slip out of alignment. I also found it possible to stick a finger inside of the back of the housing as I was trying to get the housing into the radiator neck to help jiggle thins into alignment. Once everything is pushed into position carefully work the clip into position and get it fully seated then reattach the lower hose and tighten the clamp.

Next begin refilling the system. I took a large squeeze bottle with a foot long piece of small diameter hose and filled the system through the radiator cap inlet. Took a little while and I continually squeezed the hoses to try and get air out. Once full I capped the system off with the new cap.

After everything is full but before you put all of the plastic back on run the bike around the block a few laps and get it fully warmed up to check if the temp is running as it should. If it seems to be running a little hot, squeeze the hoses (wearing gloves) to move air bubbles to the top and then when it is cool enough remove the radiator cap and top the system again. Also ensure the overflow tank is at the right level.

The old cap looked to be in good shape but since I didn't find a problem with the thermostat I now began to suspect the cap. Of course if I had tested the cap first I could have saved time and money but then I wouldn't know as much as I do now.

The system is now working fine. When the heat does come up, the fans come on the needle does not go into the red zone and soon begins heading back down.

Still didn't know exactly what caused the problem yet but I know that pressure is critical in a cooling system. An increase in pressure, controlled by the radiator cap, raises the boiling point of the fluid in the system. Too little pressure and the system boils over, too much pressure and something might just blow. Took the old radiator cap to the auto parts store today and asked to use the radiator pressure tool set (they loan specialty tools at many auto parts stores, just ask). Got the blank look from the clerk but eventually convinced him that such a thing existed and that I could safely use it right there in the store. Found the kit (had to show him) and located the test pump and adapter for the small radiator cap. Our cap is labeled for a pressure of 1.5 Bar (a bit less than 22 PSI). The cap should hold pressure to just below the marked limit and should release pressure at and above that level (how fluid gets back into the overflow tank). I pumped up the pressure and it went right past 22 PSI to the max of 30 PSI (as high as the pump goes) and held firm...ah ha! My system was overheating because it could not release the pressure. Lucky I didn't blow a hose off.

And the lesson of the day is, start by checking the easy things. Fluid, obstructions, radiator cap pressure. Then go on to worrying about the thermostat.

Photos attached are to show the difference between the new and old radiator cap (new on left). Thermostats in cold position and hot position (new on left), and the fully extended measurement of the piston.

Good luck all!
Did you put the old cap back on to confirm that the overheating returned? From what I read, I don't think you can be sure that the cap was the problem. It may have been the dirt in the radiator fins. I don't see how a cap that doesn't release pressure will cause overheating.

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post #18 of 33 Old Jun 17th, 2015, 8:34 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

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This thread has me baffled. I can understand how a faulty radiator cap can lead to boiling over if it didn't maintain enough pressure to raise the boiling point past where it would at the running temps of the bike but maybe one of the engineers here can enlighten me to how a cap that doesn't release pressure would cause overheating ( not boil over as I don't recall that being the issue) the indicator would run up into the red, no reported loss of fluid. If the T-Stat is working properly the cap would normally vent at the specified pressure from expansion of the liquid ( along with any air ) and then as it cooled it would draw fluid back in with the contraction of the liquid keeping the system full as long as the reservoir had reserve in it.

What is the process that would cause the temp to climb ( all other components being reported normal) if the cap didn't vent at the specified pressure? I would think the worst that would happen is a hose would burst, not overheating.

Some other process must be at work in this case.
Well, I am an engineer and I don't see how this can be the case either. Not releasing pressure means the closed system will be able to operate at an even higher pressure than normal and thus the boiling point of the coolant will be even higher. This allows more heat to be transferred to and rejected by the radiators and thus the engine is less likely to overheat.

So, I agree with you that some other process is likely at work here, but I certainly don't know what it is. I will offer my best guess. My guess is that a failing cap may be not only not releasing pressure at the value it should, but may also not be allowing coolant to be drawn back into the engine from the overflow tank. If this were the case, then the coolant level in the engine would gradually decrease and this would cause overheating.

If someone can come up with a scenario where a higher pressure in the system causes overheating, I would also love to hear it. I just don't see how this is possible.

EDIT: Since the BMW thermostat appears to be fairly different from a typical auto thermostat, I wonder if it is somehow pressure sensitive? If an overpressure condition could cause the thermostat to bind or for some reason not open fully, that would certainly cause overheating. Seems somewhat of a stretch, but I don't know exactly how the LT thermostat is constructed and whether a higher than normal pressure could somehow cause it to not operate properly.

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post #19 of 33 Old Jun 17th, 2015, 8:46 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

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the radiator cap is set to a pre-determined psi as the water heats up it also builds pressure, as the pressure builds up it release (both heat and water) into the overflow bottle, if the cap does not release then the temp inside the radiator will climb causing over heating of the engine. yes the pressure will increase but the water temp also goes up as well, with a cap that does not release over heating will kill the engine well before a hose lets go.
This is not correct. The amount of heat carried out by the small release of coolant into the recovery tank is insignificant in regard to cooling the engine. This article explains fairly well how having higher pressure in a cooling system is a good thing and actually helps prevent overheating due to localized steam formation at the hotter spots in the engine.

IF, and I am still not convinced, the LT is really overheating due to a radiator cap that is holding pressure to a higher level than intended, then some other phenomena is coming into play.

Beating the Heat: Advantage of a High Pressure Radiator Cap | Tuner University

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post #20 of 33 Old Jun 18th, 2015, 4:00 am
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Re: Radiator Caps, Thermostats, and Overheating Info

well who feels like a idiot now

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and no koalas are not Bears



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post #21 of 33 Old Jun 18th, 2015, 6:38 am
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Re: Radiator Caps, Thermostats, and Overheating Info

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well who feels like a idiot now
Nobody should. This is a perplexing problem and one worth sorting out.
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post #22 of 33 Old Jun 18th, 2015, 8:56 am Thread Starter
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Re: Radiator Caps, Thermostats, and Overheating Info

OK, so to sum up where we are...

1) many of us appear to have solved overheat conditions by replacing the radiator caps; this has been reported many times and I was attempting to make sense out of why

2) my cap definitely was not releasing pressure and the problem was resolved after replacing the cap (no I did not put the old cap back on to recreate the problem to confirm)

3) we have not yet confirmed if others who resolved their overheat problem by replacing their radiator caps tested them and found them releasing at too low or too high pressure

4) the consensus is that a cap that is not releasing pressure should allow the fluid temp to increase even higher without boiling

Does the additional pressure impact the cooling capability of the system to the point where the temperature will continue to increase causing the overheat? Does the system need to vent the excess pressure to allow it to cool efficiently?

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post #23 of 33 Old Jun 18th, 2015, 9:44 am
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Re: Radiator Caps, Thermostats, and Overheating Info

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OK, so to sum up where we are...

1) many of us appear to have solved overheat conditions by replacing the radiator caps; this has been reported many times and I was attempting to make sense out of why

2) my cap definitely was not releasing pressure and the problem was resolved after replacing the cap (no I did not put the old cap back on to recreate the problem to confirm)

3) we have not yet confirmed if others who resolved their overheat problem by replacing their radiator caps tested them and found them releasing at too low or too high pressure

4) the consensus is that a cap that is not releasing pressure should allow the fluid temp to increase even higher without boiling

Does the additional pressure impact the cooling capability of the system to the point where the temperature will continue to increase causing the overheat? Does the system need to vent the excess pressure to allow it to cool efficiently?
Steve, everything I know about physics and thermodynamics suggests the answers are "no" and "no." However, I am not disputing your experience or that of the others. I just think something else is at play here.

Higher temps allow the radiators to transfer more heat from the coolant to the environment as heat transfer rate is a function of delta T, the difference in temp between the heat source and heat sink. Higher pressure allows higher coolant temps before boiling occurs and thus allows higher heat transfer rate and less chance of engine overheat.

A quick search will find many references to the use of higher pressure radiator caps in cars to lessen overheating problems. This is effective right up to the point you blow a seal or hose.

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post #24 of 33 Old Jun 18th, 2015, 10:40 am
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Re: Radiator Caps, Thermostats, and Overheating Info

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Well, I am an engineer and I don't see how this can be the case either.
Even though I am not an engineer, I did stay and a Holiday In n Express. I think it also helps that my dad was a rocket scientist.

One possible scenario is that the tester did not have the right fittings and did not perform the pressure test properly. But I suspect that there would have been outflow to the reservoir if the overheating continued.

Is the only indication of overheating the needle in the red when at slow speed? No fluid loss? When moving faster, the fans are not required as the forced air is likely more than the fans produce alone.

I suspect that the shower of "small particles" from the radiator forced out by air was enough to decrease the cooling capacity of the radiators enough so at low speeds, not remove as much heat as the engine was creating thus the needle rises.

They figure these cooling systems pretty close to just what is needed to keep things running under most situations so scale build up or clogged fins can decrease the overall capacity enough to cause an issue such as this. I think this may have been an air flow issue or possibly the T-Stat at the root but we can't test that in this case since so many things were done all at once. The good thing is, the issue is resolved and the ride can continue.


If someone who has had this overheating issue and replaced a cap can chime in on fluid loss/boiling over or not, that would be helpful to me as I haven't experienced this yet.


Just my 2 cents.

Gordon
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post #25 of 33 Old Mar 14th, 2016, 9:23 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

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Thanks, that makes the time I spent writing it up worth the effort. Glad to have helped.



Steve

Indeed, great write-up. I will start with the radiator cap.

Jerry


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post #26 of 33 Old Mar 14th, 2016, 9:43 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

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Even though I am not an engineer, I did stay and a Holiday In n Express. I think it also helps that my dad was a rocket scientist.



One possible scenario is that the tester did not have the right fittings and did not perform the pressure test properly. But I suspect that there would have been outflow to the reservoir if the overheating continued.



Is the only indication of overheating the needle in the red when at slow speed? No fluid loss? When moving faster, the fans are not required as the forced air is likely more than the fans produce alone.



I suspect that the shower of "small particles" from the radiator forced out by air was enough to decrease the cooling capacity of the radiators enough so at low speeds, not remove as much heat as the engine was creating thus the needle rises.



They figure these cooling systems pretty close to just what is needed to keep things running under most situations so scale build up or clogged fins can decrease the overall capacity enough to cause an issue such as this. I think this may have been an air flow issue or possibly the T-Stat at the root but we can't test that in this case since so many things were done all at once. The good thing is, the issue is resolved and the ride can continue.





If someone who has had this overheating issue and replaced a cap can chime in on fluid loss/boiling over or not, that would be helpful to me as I haven't experienced this yet.





Just my 2 cents.

I have noticed the temp gauge moving closer to the red on traffic stops or traffic alone. Runs normal otherwise. The fans come on when approaching red, and I never saw the light. Still, I will replace the radiator cap, and see what happens. I only have my LT for 6 months, but the bike is a 99 model. Who knows if it was ever replaced. One thing I noticed when I pointed a flashlight through the fairings: a huge wasp nest/house! Since I am in the process of replacing other parts, I will post a picture, and report back about the temperature.

Jerry




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post #27 of 33 Old Mar 15th, 2016, 7:46 am
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Re: Radiator Caps, Thermostats, and Overheating Info

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I have noticed the temp gauge moving closer to the red on traffic stops or traffic alone. Runs normal otherwise. The fans come on when approaching red, and I never saw the light. Still, I will replace the radiator cap, and see what happens. I only have my LT for 6 months, but the bike is a 99 model. Who knows if it was ever replaced. One thing I noticed when I pointed a flashlight through the fairings: a huge wasp nest/house! Since I am in the process of replacing other parts, I will post a picture, and report back about the temperature.

Jerry




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Jerry, it looks like you are trying to fix a problem that is not there. Sounds like normal operation to me. The only time I had the warning light come on was during slow going at high altitude pulling my camper trailer up a long grade in the mountains of Colorado. No recurrence in the past 6k miles.

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Re: Radiator Caps, Thermostats, and Overheating Info

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Jerry, it looks like you are trying to fix a problem that is not there. Sounds like normal operation to me. The only time I had the warning light come on was during slow going at high altitude pulling my camper trailer up a long grade in the mountains of Colorado. No recurrence in the past 6k miles.



Robert

Thank you, Robert!

Perhaps I should wait. I was just concerned with the approaching summer. Little by little I will get to know more about this bike. I keep reading all recommendations and posts, and with the age of the bike, and no service history, I will need to play it by ears.

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post #29 of 33 Old Mar 15th, 2016, 1:29 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Fun thread. I do feel the cooling system on these bikes is right at the margin of good enough. And I can not emphasize enough that these radiators need a good cleaning once and awhile.
Would be interesting to know what a new radiator cap pressure test results are on the original test device.
And I also have heard many times problems solved with new caps on these bikes. On the newer K bikes it is generally the radiator that is clogged or poor refill technique, which is not a concern on the older bikes.

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post #30 of 33 Old Mar 15th, 2016, 6:03 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Actually we take cooling systems for granted. Since we are talking coolant I have attached this tech. article on coolant as related to BMW motorcycles. It is very in depth and I learned stuff I would never have thought of about proper coolant procedures, maintenance and products. So if your are into this kind of thing and a little bored enjoy.

Classic BMW* K bike Coolant Change and reducing heat in the fuel tank and on the rider

Scott
2007 LT
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post #31 of 33 Old Mar 15th, 2016, 6:25 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

Can Harley Davidson radiator coolant be used in the K1200LT without damaging the cooling system?

Just Wondering...

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post #32 of 33 Old Mar 15th, 2016, 7:41 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

For the new guys I'll post this again on temperatures (especially for the pre-05s).

Thermostat opens at 185
Fans kick on at 221
Overtemp light on at 239

When the gauge is close to the red it is about 221

On the 05 and up the LCD display does not go very far above mid range (1/4") before the fans cut in.

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post #33 of 33 Old Mar 15th, 2016, 7:44 pm
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Re: Radiator Caps, Thermostats, and Overheating Info

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Can Harley Davidson radiator coolant be used in the K1200LT without damaging the cooling system?

Just Wondering...

Rob, 2000LT
Navarre, FL
Yes but it is probably way more expensive (it is only used in the VROD) than the BMW coolant from the BMW car dealer. (Same stuff the bike dealer charges double for). I.

John
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