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.
2002 K1200LT with 52K miles. Mine since new and the cooling system has always been serviced by a dealer every two years.
-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
Step 1. Buy parts
Thermostat 126.96.36.1994.985, $44.91
O-Ring 188.8.131.524.984, $5.59
Thermostat plastic cover 184.108.40.2064.879, $6.65
Fixing clip 220.127.116.114.991, $3.83
Radiator cap 18.104.22.1685.523, $33.58 (This is a new part number from the original 22.214.171.1244.983 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!
US Army (Ret)
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.