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Thread: Fuel pressure

  1. #1
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    Fuel pressure

    Iím trying to diagnose an issue where engine really stutters on acceleration. It is idling okay but doesnít want to rev. My fuel pressure on both control and primary is low but adding shims is not helping. I replaced my fuel pump with the new style thinking maybe the old pump just wasnít putting out enough pressure.

    Iím checking a few other things and am wondering if my WUR has a dirty screen or is this the screen itself. It doesnít look like other pictures Iíve seen.

    5740044F-4A69-41E6-A413-948AAE896BC5.jpg

  2. #2
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    Have you checked/replaced the fuel filter. I haven't had much luck with the shims either.

    Is this a problem that just started? Or are you working on a car that's been sitting?

    There are several screens stacked up in there. You might want to remove them with a pick. If the screens are clogged, you will have high control pressure.

  3. #3
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    I replaced the fuel filter today. Iím only getting 5 bar directly at the fuel pump. And about 4.5 bar where the line enters the fuel distributor. So I think that tells me the fuel pump is not strong enough, even though it is new. Itís the AC Delco MU1777. And the accumulator, filter, and lines are restricting a bit of pressure. So it isnít going to matter how many shims go in if the base pressure isnít even high enough.

    Car sat the last two summers. I drained and cleaned the fuel tank, too.

  4. #4
    Administrator Ron's Avatar
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    Something seems wrong here -- The pressure at the FD should match the pump pressure. (The accumulator, filter, and lines might restrict flow, but not pressure to the FD).

    Also, with 4.5 Bar at the FD, the control pressure being low wouldn't have anything to do with the pump...
    So I would suggest checking
    for leaks, the fuel line routing, and double checking the pressure at the pump (and replace it if not >6 Bar before going further).

    FuelHose2.jpg

  5. #5
    '82 T3 FABombjoy's Avatar
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    You can also leak past fuel the accumulator seal to the return line which would present similarly. That is, higher pressure at pump than at fuel distributor. This is a failure mode not discussed much but happens with time.
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  6. #6
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    Quote Originally Posted by FABombjoy View Post
    You can also leak past fuel the accumulator seal to the return line which would present similarly. That is, higher pressure at pump than at fuel distributor. This is a failure mode not discussed much but happens with time.
    Iíve ordered an accumulator as this one is about 15 years old, so could be an issue. I also think something must be wrong with the new fuel pump itself since Iím not seeing great pressure there. Battery is new and voltage is good at the fuel pump.

  7. #7
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    Quote Originally Posted by Ron View Post
    Something seems wrong here -- The pressure at the FD should match the pump pressure. (The accumulator, filter, and lines might restrict flow, but not pressure to the FD).

    Also, with 4.5 Bar at the FD, the control pressure being low wouldn't have anything to do with the pump...
    So I would suggest checking
    for leaks, the fuel line routing, and double checking the pressure at the pump (and replace it if not >6 Bar before going further).

    FuelHose2.jpg
    Not seeing any leaks and nothing on the ground. I havenít messed with fuel lines(except the one for pressure testing) so they are all routed correctly. I agree, it seems the fuel pump might be the issue even though itís new. Any recommendations on a better replacement? I donít want to go back to the old style.

  8. #8
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    Double check the two hose connections on the fuel pump, maybe they got switched around?


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  9. #9
    Senior Member 82DMC12's Avatar
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    No reason to change the accumulator unless you fail the "rest pressure test". Also you can remove the small line on the rear of the accumulator, plug the rubber hose, jump the RPM relay, and see if fuel is dribbling out of the accumulator where the hose used to go. If you are dripping, change the accumulator. If it's not dripping, it's fine, and your problem is somewhere else.

    Where did you get the part # for the fuel pump? I have a new pump from a vendor but it's a Delphi.

    Change the fuel filter, check the accumulator as I explained, and then re-test the primary pressure. Check the shop manual to see how to put the pressure tester in line with the CPR. It's easy to get it backwards in regards to the bypass that shows primary/control pressure. The instructions that come with these testers do not supersede the DMC shop manual.
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  10. #10
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    Quote Originally Posted by 82DMC12 View Post
    No reason to change the accumulator unless you fail the "rest pressure test". Also you can remove the small line on the rear of the accumulator, plug the rubber hose, jump the RPM relay, and see if fuel is dribbling out of the accumulator where the hose used to go. If you are dripping, change the accumulator. If it's not dripping, it's fine, and your problem is somewhere else.

    Where did you get the part # for the fuel pump? I have a new pump from a vendor but it's a Delphi.

    Change the fuel filter, check the accumulator as I explained, and then re-test the primary pressure. Check the shop manual to see how to put the pressure tester in line with the CPR. It's easy to get it backwards in regards to the bypass that shows primary/control pressure. The instructions that come with these testers do not supersede the DMC shop manual.
    I got the part # from another post on this forum. I believe the AC Delco MU1777 and the Delphi FG0089 are basically the same from what I can find online, but maybe something is different internally. Iíll check the accumulator as you suggested but might be the weekend before I get back to this. When I wasnít getting the right pressure at the FD, I connected the gauge directly to the pump at the tank and was only seeing 5 bars there, so Iím not starting from a good pressure.

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