Low fuel pressure… even after…

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  • #987424
    Michael Woosley
    Michael Woosley

    Hello everyone. I like to consider myself a decent mechanic having been one most of my life. A buddy of mine has an issue with his 95 Olds 88 that has me stumped like crazy.
    He inherited the car from his father-n-law a couple years ago. The reported problem is a rough idling engine, generally after engine warmup. These are the things that I’ve diagnosed/replaced with ZERO satisfactory improvement.
    #1 – Connected fuel pressure tester to fuel rail, measured ~25 psi
    a. What we did… because the fuel pressure rating is 47-55 psi, we believed a pump replacement would solve this particular problem… it DID
    NOT, as we did not achieve the desired rated psi.
    b. We tested the ignition module at the local parts store, it tested good. My buddy replaced it because he heard that it was a potential cause
    for the stumbling engine performance… no change in issue.
    c. I inquired about a tune up and was told it was done a few hundred miles before.
    d. I had much to do, so my buddy decided to have a couple local shops look at it. Neither could determine the cause of the stumbling,
    although one of them said they swapped in a PCM and it seemed to improve, but not much.
    e. So, here I am again trying to help him figure out this problem.
    f. I convinced him to replace the upper and lower intake gaskets in order to eliminate a potential source of vacuum leaks. This allowed us to
    get at each and every vacuum line and fitting. A couple were loose on their hard plastic line, but again, no improvement.
    g. We pinched off the fuel return line to see if we could get an increase in the fuel pressure. We did see an increase; to 65 psi. This led
    me to believe that the fuel pressure regulator was malfunctioning… so we replaced that… STILL no improvement.
    h. Because the first pump we put in was of the “budget friendly” persuasion, we figured what the heck, lets replace the fuel pump again.
    After doing so, we did SEE AN IMPROVEMENT, but only to 34 psi… a 9 psi increase… BUT STILL no where near our 47-55 psi specification.
    When we pinch off the return line now, the pressure jumps to at least 100 psi, indicating a much stronger pump.

    So now I’m absolutely lost in the sauce… We believe we have performed all the necessary diagnoses’ correctly, replaced more than enough service parts, but still have a drivability issue. Tonight, I noticed that the fuel pump would not come on a couple times when the key was turned to KOEO position. We are going to look at the fuel pump relay to see if the contacts are potentially burnt/corroded; preventing a good connection.

    Other than that, have any of you experienced this type of problem. Obviously, I could have a problem with a ground or 2, but my thoughts are that because the engine starts, runs and drives, the grounds are an unlikely problem… we’ll see I guess.

    Thanks ahead of time for any suggestions.

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  • #987524

    Have you checked or replaced the fuel filter?

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