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Thread: Sudden drop in fuel pressure

  1. #1
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    Sudden drop in fuel pressure

    I have a friend that is struggeling with a problem.
    The problem is that you let the car idle, the fuel pressure will be stable at 3.5 bar, then suddenly drop to 2.5 bar in 30 seconds, and stay there. Switch off the ignition and back on, and pressure is 3.5 bar again.
    But after 30 sec or so, its down to 2.5 again.
    Regulator and pump has been changed.

    Anyone have any inputs?

  2. #2
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    Have the regulator and pump been changed prior to the issue appearing or after?
    "If you have integrity, nothing else matters. If you don't have integrity, nothing else matters." Alan K. Simpson.
    5spd E46 "3XX": 328i engine, 330i Intake and exhaust, CAI, 323i diff.

  3. #3
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    I'd question the gauge.

    PLEASE always tell us exactly what car we're dealing with...... (Maybe the pressure's actually SUPPOSED to be 2.5 bar....we don't know, if we don't know what car you're talking about.)

    Chris Powell
    Racer and Instructor since, well. decades, ok?
    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  4. #4
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    He bought the car with a hesitatation problem.
    Checked the pressure, with the same result. With 2 diffrent gauges.
    Then changed pump and fpr, with no change in pressure.

    It sounds like the pump is suddenly dropping speed, when the pressure drops.
    But the pressure is really stable at 2.5 or 3.5 bar. It just drops suddenly 1 bar. Without fluctuating.

    Car is a subaru. Hope thats ok.
    Could it be some sort of speed controller for the pump?

  5. #5
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    Pumps typically run at a single speed. A sudden drop can be caused by any of the following:
    - a partially blocked fuel filter
    - a soft failing fuel pump
    - a blockage in the fuel line.

    I have no idea where the fuel pump is located in a subbie (Chris should be able to help!) but I’d listen to it to see if the noise it makes changes.

  6. #6
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    Quote Originally Posted by s50b30 View Post
    He bought the car with a hesitatation problem.
    Checked the pressure, with the same result. With 2 diffrent gauges.
    Then changed pump and fpr, with no change in pressure.

    It sounds like the pump is suddenly dropping speed, when the pressure drops.
    But the pressure is really stable at 2.5 or 3.5 bar. It just drops suddenly 1 bar. Without fluctuating.

    Car is a subaru. Hope thats ok.
    Could it be some sort of speed controller for the pump?
    Given what's in green it seems like this pressure drop is commanded by the ECU for some reason, like some sort of limp mode. Does the pressure drop after 30s EVERY time, even after a warm startup, or only after a cold startup (and then stays at 2.5 once the engine is warmed up?).

    I'm not knowledgeable enough to take it further at this point, but I'd say look at what changes in engine management conditions after 30s. Could be a table/table switch triggering this; for e.g. the pressure is at 3.5 as long as fuel management is being controlled by a "cold startup enrichment" table, and then drops.

    Dumb question maybe: is it really a problem if the pressure drops to 2.5? Or maybe it's supposed to be that way and the "hesitation problem" comes from something else? Does the pressure go up when pushing the throttle or is it stuck at 2.5?
    Last edited by Breeze1; 08-03-2018 at 10:14 AM.
    "If you have integrity, nothing else matters. If you don't have integrity, nothing else matters." Alan K. Simpson.
    5spd E46 "3XX": 328i engine, 330i Intake and exhaust, CAI, 323i diff.

  7. #7
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    Given that it's a Subie (I own one, for a winter beater, btw), I'd have to say you should throw a fuel filter at it. I mean, the damned thing sits right up there, facing upwards, in the engine compartment, and it costs about a quarter. (Sure, I'm exaggerating.)

    Um, if you'll be more specific about WHAT subaru, I'll look it up on Alldata and see what I can find. I just drive mine when the weather's really rough, don't work on it any more than I have to....

    Chris Powell
    Racer and Instructor since, well. decades, ok?
    Master Auto Tech, owner of German Motors of Aberdeen
    BMWCCA 274412
    German Motors is hiring ! https://www.bimmerforums.com/forum/s...1#post30831471

  8. #8
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    Well. The regulator is 100% mechanical, so pressure should be the same regardless.
    Filter has been changed aswell.
    This turned out to be quite the headache.

  9. #9
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    Is there a vacuum hose on this Subaru FPR? Is it connected to the correct spot?

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  10. #10
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    So after a little more research we found out that fuel pressure is too high the first 30-40 seconds. Not to low.
    The same things happens with the vacuum hose to fpr disconnected.
    Im even more puzzled now.

  11. #11
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    The FPR is bad, I bet. But, why would it revert to the correct pressure after 30 seconds of operation??? Time for a Haitian Voodoo Priestess!!!

    Chris, what’s your guess??

  12. #12
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    It did the same thing before changing the fpr with a new one.

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