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  1. #1
    Cadet
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    Default What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    My boat is at the shop after experiencing loss of power and WOT of only 3000 RPM. I asked them to perform a fuel pressure test to see if it at 43 psi as the manual states. They are claiming that the only way to get an accutare fuel pressure reading it to take it to the lake and run it at WOT. This is really starting to irritate me since the boat is in the shop because it won't reach WOT. So, should they be able to get 43 psi by putting a tester on the fuel rail at idle or or they BSing me?

  2. #2
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    They are 100% correct. You have a problem of not getting to WOT, so what good is checking the pressure at idle? If it drops significantly while attempting to reach WOT, it will give them a direction to continue troubleshooting.
    If you don't want to allow them to do the testing, then pick your boat up buy the tools and manuals and do the testing yourself. But don't tie their hands so they can't test, then complain because it took them so long to find the problem.
    Don S.


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  3. #3
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    My thought was that the fuel pressure and tests that can be done in the shop should tell us something. I have read in these forums that the fuel system test outlined in the ECM 555 manual can be performed to give an indication of where the problem might be. Shouldn't they at least be running all of the possible in-shop tests before taking it to the lake. They are stating that this lake test will involve two techs at 2 hours each for a total of 4 hours and $400 bucks ouch!

  4. #4
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    Maybe they did test it already and it was 43. Did you ask them?

    For $400, you can buy your own gauges and take the boat to the water yourself to test the pressure.
    I have hooked gauges to the engine like that for customers in the past so they didn't have to pay for the all the driving and launching/retrieving and testing time. Or perhaps offer to take a tech to the water and drive the boat while he does the testing. I've done this with customers also.
    Don S.


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  5. #5
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    I asked the service manager what the psi was at idle and he didn't know because the techs had all of that information. I also suggested that I take the boat out and do the test but he didn't sound too crazy about that. He stated that he would go talk to his techs and see what they thought. I do like the idea of me driving while a tech does the testing better than paying two techs at 2 hours each. I picked up a fuel pressure tester at lunch and I might just get the boat back and start testing things myself. So, would it be normal to see a psi of 43 at idle on this engine?

  6. #6
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    I got the boat back and ran a fuel pressure test from the shrader valve on the fuel rail with the following results:

    PSI with key turned on and fuel pump engages: 43
    PSI with key turned on and fuel pump shuts off: 40 steady with no drop off over time
    PSI with engine idling: 38

    Does all of this sound normal? The manual states that this engine should be getting 43 PSI but is that at idle or under load?

  7. #7
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    Have you read this in the manual?





    Service bulletin 99-7 below

    VVVVVV
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    Don S.


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  8. #8
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    Thanks for that information Don. I am reading manual #31 and I did not see this section. I am going to take the boat out tomorrow and do a fuel pressure test from the fuel rail while under load. I will post the results of that test. Thanks again!

  9. #9
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: What should fuel pressure be at idle? Mercruiser 2002 350 MAG MPI

    You need to be doing this on the water test with not only the fuel pressure gauges, but if you don't have a diagnostic tool hooked to the engine so other things can be checked at the same time, you are only doing the job half way.
    Your problem may not even be a fuel problem.
    Don S.


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