Fuel pressure regulator - BMW Luxury Touring Community
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post #1 of 8 Old May 13th, 2017, 10:41 pm Thread Starter
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Fuel pressure regulator

I think I already know the answer but how often should the regulator be open allowing fuel to return to the tank, particularly at idle? I'm doing some last minute maintenance before a trip and was double checking the hose clamps in the tank to make sure I didn't have a slight leak. I've noticed a bit of a hesitation and just wanted to verify there wasn't still an issue there. I had the fuel level sending unit out with the fuel level below the lowest hose clamp so I had a clear view. I noticed the fuel was rolling while running as fuel was being returned. I expect this to be relatively normal provided I have adequate fuel pressure to the rail. I know the proper thing to do is get a fuel pressure gauge and I might try and borrow one from the parts store tomorrow. I just thought I might save some time if it was odd to begin with.
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post #2 of 8 Old May 13th, 2017, 11:11 pm
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Smile Re: Fuel pressure regulator

From your description, everything sounds normal.
If the fuel pump is running, the fuel regulator will be returning fuel to the tank.
The regulator's job is to maintain a constant pressure in the injector rail, around 50psi if memory serves.
At idle the fuel demand is low, so fuel return will be greater than if you were under load.
Checking the pressure is a good idea. It will eliminate another variable.

Good Luck!

Duane

Check the obvious first!
01 LT Champagne "The Starship"
73 Z1 Kawi
and 7 long gone
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post #3 of 8 Old May 13th, 2017, 11:45 pm
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Re: Fuel pressure regulator

The fuel pump on the LT is a positive displacement pump so it is going to be pumping the same volume of fuel per minute regardless of fuel consumption. It will be returning through the regulator a larger amount of fuel at idle than when running but that is probably a relatively small difference over all. A pressure gauge is the only real way to verify the proper operation of the pump and regulator but you should see a constant inflow back to the tank when running. It will not be a pulsing as the pump will be forcing the same amount of fuel through the system all the time. The only difference in the return will be what the engine uses under load versus at idle. If none of the hose connections are leaking, you should be fine.

Gordon
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post #4 of 8 Old May 13th, 2017, 11:52 pm Thread Starter
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Re: Fuel pressure regulator

I was thinking that was the case but wanted to make sure I wasn't missing something. No issues at speed and plugs look good so I'm thinking fuel pressure is probably ok. I may verify it anyway, just for peace of mind.
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post #5 of 8 Old May 14th, 2017, 6:44 am
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Re: Fuel pressure regulator

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Originally Posted by St0rm1 View Post
I was thinking that was the case but wanted to make sure I wasn't missing something. No issues at speed and plugs look good so I'm thinking fuel pressure is probably ok. I may verify it anyway, just for peace of mind.
Just remember, you will need a T fitting to measure the running pressure. If you dead head the pressure line, the pressure will be much higher and could actually burst or pop off the inside tank lines.

Gordon
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1998 R1100RT Never should have sold it
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post #6 of 8 Old May 14th, 2017, 6:17 pm Thread Starter
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Re: Fuel pressure regulator

Borrowed a pressure gauge from the parts store and checked the running pressure. Even with the somewhat questionable accuracy of the gauge it never dropped below 50 psi. I ran the RPMs up and it held pretty steady. Looks like the regulator is working as needed.

I did find my NGK plugs are shorter than the Bosch plugs. There was spark jumping across the space between the plug socket and the spark plug tip. In doing so it was jumping to the socket to the engine. Installed the Bosch plugs and no more arcing. Now to replace the rear tire with a nail in it, change the fluids and start loading her up.
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post #7 of 8 Old May 14th, 2017, 7:59 pm
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Re: Fuel pressure regulator

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Originally Posted by St0rm1 View Post
Borrowed a pressure gauge from the parts store and checked the running pressure. Even with the somewhat questionable accuracy of the gauge it never dropped below 50 psi. I ran the RPMs up and it held pretty steady. Looks like the regulator is working as needed.

I did find my NGK plugs are shorter than the Bosch plugs. There was spark jumping across the space between the plug socket and the spark plug tip. In doing so it was jumping to the socket to the engine. Installed the Bosch plugs and no more arcing. Now to replace the rear tire with a nail in it, change the fluids and start loading her up.
To get accurate fuel-pressure system checks (and regulator check) you need do to a bit more than above...

(1) Assuming gauge is fairly accurate, you should get close to 50 PSI in any tests below. In case of having no other gauge to compare accuracy, I would tolerate values +/-10% above/below IF-AND-ONLY-IF the engine behavior is always good in all running RPM (including cold-start and high RPM acceleration).

(2) As a first test: engine off, gauge should be connected to a "T" at quick-connector on the pressure side hose (most inside hose at bottom of fuel-pump flange). Turn ignition ON, do NOT start. Check fuel pressure reading after 6 seconds: should be between 45-50 PSI - this is the prime fuel system pressure. Turn ignition OFF.

(3) Wait another 2 minutes and note IF gauge reading is losing pressure compare to the earlier priming pressure. It is a close-system and should maintain pressure for many minutes.

(4) Do same pressure reading test as above with engine running at various RPM (engine cold and hot). Of course, as it is nearly impossible to ride with this gauge installed, you have to limit the kind of RPM you can test in garage with fuel-pressure gauge connected.

(5) Shut Engine OFF, but leave pressure gauge connected. Come back for more readings every 2 minutes for about 5 times. Note how much pressure system is losing with engine OFF over time. It should not drop below 42 PSI after a 10 minutes delay.

A drop of pressure with engine stopped (test 2-3 OR in last test) could be caused by a bad fuel-pressure regulator OR a leaking/stuck fuel injectors OR a cracked/leaking fuel-hose inside tank.

ATTENTION: do NOT ever turn ignition ON or attempt to start engine UNLESS both hoses are fully connected under fuel tank (both quick fuel connectors). You need a path for the returning fuel supply from regulator (at fuel-rail), otherwise the system is being over pressurized until the over-pressure valve at fuel-pump opens up as an emergency path (assuming it does function).
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post #8 of 8 Old May 15th, 2017, 8:26 am Thread Starter
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Re: Fuel pressure regulator

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Originally Posted by sailor View Post
To get accurate fuel-pressure system checks (and regulator check) you need do to a bit more than above...

(1) Assuming gauge is fairly accurate, you should get close to 50 PSI in any tests below. In case of having no other gauge to compare accuracy, I would tolerate values +/-10% above/below IF-AND-ONLY-IF the engine behavior is always good in all running RPM (including cold-start and high RPM acceleration).

(2) As a first test: engine off, gauge should be connected to a "T" at quick-connector on the pressure side hose (most inside hose at bottom of fuel-pump flange). Turn ignition ON, do NOT start. Check fuel pressure reading after 6 seconds: should be between 45-50 PSI - this is the prime fuel system pressure. Turn ignition OFF.

(3) Wait another 2 minutes and note IF gauge reading is losing pressure compare to the earlier priming pressure. It is a close-system and should maintain pressure for many minutes.

(4) Do same pressure reading test as above with engine running at various RPM (engine cold and hot). Of course, as it is nearly impossible to ride with this gauge installed, you have to limit the kind of RPM you can test in garage with fuel-pressure gauge connected.

(5) Shut Engine OFF, but leave pressure gauge connected. Come back for more readings every 2 minutes for about 5 times. Note how much pressure system is losing with engine OFF over time. It should not drop below 42 PSI after a 10 minutes delay.

A drop of pressure with engine stopped (test 2-3 OR in last test) could be caused by a bad fuel-pressure regulator OR a leaking/stuck fuel injectors OR a cracked/leaking fuel-hose inside tank.

ATTENTION: do NOT ever turn ignition ON or attempt to start engine UNLESS both hoses are fully connected under fuel tank (both quick fuel connectors). You need a path for the returning fuel supply from regulator (at fuel-rail), otherwise the system is being over pressurized until the over-pressure valve at fuel-pump opens up as an emergency path (assuming it does function).
As always, I appreciate your feedback. As with most rental tools the life it lives is rough at best. I'd say the gauge was within the 10% accuracy range so the readings should be sufficient for green light red light test. My biggest concern was low pressure at idle or higher RPM. I had replaced the internal hoses last year and double checked the clamps Saturday to ensure no leaks. So far everything is holding together but that's when I noticed the amount of fuel return which made me wonder about the pressure.

I did most of the procedure you listed but didn't give the bleed down test as long as you listed. Primed not running the pressure came in ~53 psi. It held the pressure for a few minutes as I was messing around with a few other things. Running it was slightly higher ~56 and maintained a pretty steady reading though out the RPM range. I contemplated trying to take it for a ride to see it under load but I have a flat rear tire and didn't feel like trying to air it up, etc., etc. After shutting it off I mucked around for a few minutes before disconnecting things and it hadn't lost much pressure but I didn't pay to much attention to the amount of time or pressure. I could do the procedure again but I think it answers my primary question and if I was still concerned I'd order a gauge to ensure it's accuracy. All in all, I'd say a green light.
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