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Thread: Fuel pressure regulator.

  1. #1
    Join Date
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    Fuel pressure regulator.

    Hi all, i have a 90 850 which i tried to take out yesterday after sitting for 2 weeks. There was a strong gas smell so i took it back to my garage to investigate.
    The smell was coming from engine bay, i checked the 2 hoses near fire wall before the fuel rails and the one for left bank was soft and not holding pressure.
    I took the fuel rail out ,reconnected to fuel hose and jumped the fuel pump from relay and fuel came pouring out from bottom of FPR. All the searches that i have done shows faulty regulators leak from the vacuum side, but this regulator is definately not holding pressure. Am i correct?

  2. #2
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    Quote Originally Posted by razman850 View Post
    Hi all, i have a 90 850 which i tried to take out yesterday after sitting for 2 weeks. There was a strong gas smell so i took it back to my garage to investigate.
    The smell was coming from engine bay, i checked the 2 hoses near fire wall before the fuel rails and the one for left bank was soft and not holding pressure.
    I took the fuel rail out ,reconnected to fuel hose and jumped the fuel pump from relay and fuel came pouring out from bottom of FPR. All the searches that i have done shows faulty regulators leak from the vacuum side, but this regulator is definately not holding pressure. Am i correct?
    No you are not correct, one of your fuel pumps is not holding pressure if the hose next to the firewall is soft.
    The FPR is just doing it's job and relieving extra pressure to the return line.
    Replace the fuel pump.

  3. #3
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    The FPR bleeds off excess fuel from the bottom... that is the return hose back to the fuel tank and how the FPR regulates fuel pressure in the rail. If fuel was escaping from this line it suggests your fuel pump is achieving fuel pressure or the FPR is stuck open. You could try swapping the FPR's to see if the other side behaves the same.

    If you can smell gas in the engine bay you should be able to see it (wetness). If your fuel hoses are original I would replace all of them including by the fuel filters. You can buy this hose in bulk and cut to lengths needed.


    Quote Originally Posted by razman850 View Post
    Hi all, i have a 90 850 which i tried to take out yesterday after sitting for 2 weeks. There was a strong gas smell so i took it back to my garage to investigate.
    The smell was coming from engine bay, i checked the 2 hoses near fire wall before the fuel rails and the one for left bank was soft and not holding pressure.
    I took the fuel rail out ,reconnected to fuel hose and jumped the fuel pump from relay and fuel came pouring out from bottom of FPR. All the searches that i have done shows faulty regulators leak from the vacuum side, but this regulator is definately not holding pressure. Am i correct?
    Last edited by BMSman; 09-10-2017 at 06:59 PM.

  4. #4
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    Thanks for the replies, from what i have read it is supposed to do that when there is a vacuum under idle.
    I dont want to throw away a good regulator nor change a pump if i dont have to so i will test both.
    First i will check pump by switching fuel hoses at fire wall and if it still loses pressure means pump is good. I will do same with regulator.
    The reason i check the fuel rail this way was to check injector O rings for leaks at the rail.
    I could swear i have done this test before in the past and didnt have fuel pouring out the return side unless i applied vacuum.
    Non of hoses in engine bay and filter are original. All have been changed few years back.
    Last edited by razman850; 09-10-2017 at 07:20 PM.

  5. #5
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    Had the same recently when I installed new fuel pumps to replace the old 28 years old ones, Used a wire from B+ pole and touched without ignition on the fuse 23 and then 24 (on E32), so I could hear the fuel pumps running for a second or 2, fuel lines leaked where the hoses come up at the back of the engine and also under the fuel pressure regulators. Looks like the old pumps had much less fuel pressure as they were at the end of their lifetime. Had to retighten several fuel hose clamps then, also under the fuel pressure regulators, difficult to get there, need small tools.

    To test the fuel pressure regulator: pull off the vacuum hose from the FPR, if you see there fuel drops, replace, = diapgrahm cracked.
    Testing for a leaking, ruptured, fuel pressure regulator diaphragm
    That is a good analysis. How to verify the cause of an intermittent long crank and a start/stall condition. No need to have the electronics as shown first (because we backyard mechanics do not have them anyway), just remove the vacuum hose and sniff if there is fuel smell or even fuel coming out. Watch for fuel. Takes over a minute that fuel is coming out of the port in this vid, diaphragm cracked. https://www.youtube.com/watch?v=AKtR_yF7bi8
    To be on the safe side, I would connect on the fuel pressure vacuum connector a transparent hose and lead it out of engine bay into a container/bottle, just in case a lot of fuel is splashing out.
    Last edited by shogun; 09-10-2017 at 07:53 PM.
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  6. #6
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    Well i just tried the other rail and the fuel flows out on the return side without any vacuum. Have not run it long enough to check if it comes out of vacuum port on top.
    I did the test with same pump that seems to not hold pressure.

    The reason i hope its not the pump is i have never been able to get that damn plastic collar off in the 11 years i have owned this 850. I have a spare pump assembly but dread trying to get to the one in the tank.
    I will order 2 regulators tonight from rockauto just incase.
    thanks for all the input.

  7. #7
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    Quote Originally Posted by razman850 View Post
    Well i just tried the other rail and the fuel flows out on the return side without any vacuum.
    Of course it does... that's what it is supposed to do. All the vacuum line does is drop a little off the fuel pressure at idle/low speed - but the regulator will always pass whatever extra fuel over the set pressure... the pumps can supply far more than the system requires, hence the bleed off...

    ...and have you actually priced regulators? I wouldn't be so gung ho on replacing them if I wasn't sure...
    '91 Dinan 860 Stage III (soon to be Dinan TT 6L)
    '91 850i 6sp (mint)
    '91 850i 6sp (getting there)
    '90 Dinan 750iL TT stage III
    '94 850 CSi (being restored to factory perfect)
    '94 840ci (4 short - obvious parts car), '91 850i parts car
    '73 3.0 csi, '08 535i, '95 318i
    ...and a few other non BMW cars

  8. #8
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    Quote Originally Posted by cartoonz View Post
    Of course it does... that's what it is supposed to do. All the vacuum line does is drop a little off the fuel pressure at idle/low speed - but the regulator will always pass whatever extra fuel over the set pressure... the pumps can supply far more than the system requires, hence the bleed off
    That explanation was what i was looking for. I already ordered 2 from rockauto for about $40 each, will keep them for spares, i wouldnt have pulled the trigger on $300 oem ones that fast. Thanks again.

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