1994 Mercruiser 502 intermittent motor issue
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1994 Mercruiser 502 intermittent motor issue

  1. #1
    Junior Member
    Join Date
    Jun 2012

    Default 1994 Mercruiser 502 intermittent motor issue

    I have a 1994 Mercruiser 502 motor (fuel injected) which is having some problems which I have no idea how to resolve. The boat runs great for a good part of the day. I will turn the engine off for 30 or 40 minutes and try to start the engine. The engine will start up with no problems but it will run really bad. The motor sounds like a lean popping with no black smoke. If I feather the throttle for a min or two the motor will pick up and start running normally for the rest of the day. Someone told me that it sounds like a carbureted motor with a bad vacuum leak when experiencing this issue. Someone also told me that I had a bad Idle air motor. I replaced it and am still having the same problem.

    I had another issue as well. after running the motor for a good part of the day it wouldn't start. I had to hold the throttle open all the way and crank it to get it to start. After it started it ran perfect.

    I'm not sure why it is acting like this because when the motor is running it is running great. If anyone has any ideas that I could try I would really appreciate it. I just bought the boat and want to get this fixed.

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  3. #2
    Senior Member EVILFORCE's Avatar
    Join Date
    Oct 2009


    I remember a vapor-lock issue. Do not know for sure if yours is involved. That is waht it sounds like.
    No Fool Like A Old Fool is there?

  4. #3
    PASO ROBLES CA cougar63's Avatar
    Join Date
    Aug 2008


    I think you are correct with the vapor lock diagnosis.Also I believe pre 97 the fuel injected 502 did not use a cool fuel system and instead ran 2 fuel pumps.The diaphram in the upper pomp may be worn causing the prob. just my 2cents

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  6. #4
    Senior Member namba860's Avatar
    Join Date
    Jun 2009
    Las Vegas, Nevada

    Default 502

    Locate your vst tank on the back of the engine. On top there is a yellow tygon hose coming from it. Fire the engine up and pull that hose off and see if you have fuel coming out of it. If there is fuel coming out of it the diaphgram in the vst tank is bad. Remove the cover and flip it uside down there are three phillips screws, remove them and replace the diaphgram. The part number is #808509 for the diaphgram and the part number for the cover oring only is #803831. If you find this pull #7 and #8 spark plugs and see which one looks fouled and replace it. When the diaphgram breaks it will route the fuel from the top of the vst tank threw the tygon hose and push it in the intake plenum and the fuel will go into one of those 2 cylinders. Sometimes even hydraulicing the cylinder.when you have to advance the throttle on and efi engine it will go into whats called clear flood mode to start it. Sometime the map sensor can cause that. You would need to have the engine scaned for codes at that point.
    offshore marine

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