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Thread: Fluctuating fuel pressure - any fixes?

  1. #1
    Join Date
    Dec 2003
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    1,640

    Default Fluctuating fuel pressure - any fixes?

    The fuel pressure on my M30 will typically sit at 42 lbs. Just right.

    Occasionally after the car has been running a while, the FP will drop to about 35 or 36 lbs and stay there. At least until the car is off and sits for a while.

    The lower FP messes up the mixture and the car runs a bit leaner than I would like, especially on idle.

    Any ideas short of replacing the fuel pump? The FPR has been swapped with a different unit and the behavior is consistent so I don't think it's the FPR.

    Ideas?

    Jeff
    Bellevue WA
    90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
    86 Porsche 951 - Track Toy

  2. #2
    Join Date
    Feb 2007
    Location
    New Forest, ,UK,
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    Default

    I know it may be teaching your grandmother to suck eggs but that sounds like a blocking filter.

  3. #3
    Join Date
    Jan 2004
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    Default

    Quote Originally Posted by Jeff N.
    The fuel pressure on my M30 will typically sit at 42 lbs. Just right.

    Occasionally after the car has been running a while, the FP will drop to about 35 or 36 lbs and stay there. At least until the car is off and sits for a while.

    The lower FP messes up the mixture and the car runs a bit leaner than I would like, especially on idle.

    Any ideas short of replacing the fuel pump? The FPR has been swapped with a different unit and the behavior is consistent so I don't think it's the FPR.

    Ideas?

    Jeff
    Hey Jeff...

    Did you replace it with another adjustable one or try the stock / 944 variety too?

    I would think the FPR's job is to keep it level regardless of the system variances... and if it's changing then the FPR is not doing it's job.

    Could your vacuum to the FPR be changing / high-temp vacuum leak?

    This is measured with the gauge that lives under the hood? I wonder if the gauge gets off calibration when hot.

    So in summary... no answers, just more questions
    Robin

    72 Chevy K10
    01 E39 M5

  4. #4
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    Jan 2004
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    Benneton (United Colors of)
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    a quart of marvel mystery oil in 10 gallons of gas seems to make fuel pumps really happy to be alive. i don't think that anyone makes a specific fuel pump rejuvenator (sp) that works as well as MMO. that, and a fuel filter change.
    "..Torchinski v. Peterson that it is legal to carry a concealed weapon, so long the weapon is totally slick like a huge ass machine gun that you carry under a trench coat, like in the Matrix."


  5. #5
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    Dec 2003
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    Quote Originally Posted by Robin-535im
    Did you replace it with another adjustable one or try the stock / 944 variety too?
    It's just a stock 3 bar unit. The adjustable one is on the bench as I was having the same problem with it.

    Quote Originally Posted by Robin-535im
    I would think the FPR's job is to keep it level regardless of the system variances... and if it's changing then the FPR is not doing it's job.
    Right. Except why would it happen on two different units?




    Quote Originally Posted by Robin-535im
    Could your vacuum to the FPR be changing / high-temp vacuum leak?
    Now that's an idea. As I think about it...as the vacuum goes to zero, the FP rises. If you pull the vacuum line when things are working right, the FP goes to about 47. So, low FP wouldn't be related to a leak I think.


    Quote Originally Posted by Robin-535im
    This is measured with the gauge that lives under the hood? I wonder if the gauge gets off calibration when hot.
    Yes. I don't think the gauge is off because I can tell I have a problem via the wideband gauge in the cabin. The car starts to idle like crap and the mixture goes lean. If I then open the hood and check the FP, it's low.

    I suppose I could put a new filter in it...
    Bellevue WA
    90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
    86 Porsche 951 - Track Toy

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