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Thread: 7.4 MPI no start

  1. #1
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    7.4 MPI no start

    I've got a 2000 7.4 MPI that won't start. I'm leaning toward a fuel issue. Here's what's going on:

    Uncovered it from the shrink wrap earlier this spring. Put it on the hose and could barely keep it running and when it did the rpm would rise and fall like it had a big cam. I heard the IAC opening and closing like crazy so I thought that may not be operating properly. Frantically threw a new IAC in it trying to get on the water. Ran a bit rough, but ran. Got to to the lake and nothing.

    If I cycle the fuel pump by turning the ignition on and off several times without bumping the starter it will fire for a brief moment. The pump is definitely spinning but at what pressure I dunno. I've checked simple things like my Racer fuel filter, drained and topped with fresh fuel. Replaced the worn vacuum line on the fuel regulator. Previously when I'd open the fuel filter id need to purge any air from the lines using the port just behind the thermostat housing. There's no air in the lines, I'm pretty sure.

    Just today I bought a fuel pressure test kit. it was kinda late so I couldn't start it up but turned the ignition switch on to cycle the fuel pump. 35-45 psi several times. Where do I go next? I was almost completely convinced it's a fuel issue but not so much anymore. Tomorrow I'll attempt to start it again while monitoring the pressure. How can I determine if it's a pump vs a regulator issue?

    Thanks

  2. #2
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    oil pressure safety switch? shouldn't the fuel pump only pump when you are cranking then it should only pump when there is oil pressure?
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

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    Are the oil pressure safety switch and sender for the gauge separate?

    The gauge is reading pressure

  4. #4
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    that's probably a question for a merc guy with my electric fuel pump I have no other electronics so I have an OP sender and an OP switch the OP switch sends voltage to the fuel pump when I am cranking then when I release the key there is no voltage until oil pressure builds. on an EFI boat that function may be handled by the ECM. If you have clips on your test leads of your digital multimeter I would check for voltage at the fuel pump while cranking.

    don't some of the merc boats have an electric safety valve in the fuel line from the tank to the motor. I seem to remember hearing a buzz from that when the key is turned to on when we put the 383 in the the sting.
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

  5. #5
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    I'm defenitley heading that way. Thanks for the lead. Of course someone is borrowing my Merc manual right now but I think I can figure it out with google and a parts diagram. I'll let you know what i find

  6. #6
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    Best diagnostic trick out there - Spray a little Gumout/carb cleaner(not ether/starting fluid which pings badly) down the flame arrestor. If it is fuel issue it will run perfectly for a few seconds, if not it is ignition. Works on everything from lawnmower on up.
    Cheers,
    Pismo
    1996 22 Classic
    Red with Stainless Windshield
    Stock Gen VI 502 Magnum MPI-415hp
    Stock Bravo I
    25" Mirage Plus
    74.5mph best @ 5050rpm GPS (Speedo said 80)
    27" Labbed Mirage Plus
    75.5mph best @ 4800rpm GPS (Speedo said 82)

  7. #7
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    research mercruiser mpi "cool fuel" system.

  8. #8
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    What exactly did you mean by, drained and topped with fresh fuel? Did you drain the tank, or the fuel filter? This sounds like a bad fuel case.
    That port behind the thermostat housing is the on the end of the fuel rail. It is for checking the fuel pressure with a gauge, not for purging air. I would start with checking the pressure, which you did and it looks good. Next i would be getting a new source of fuel, like a temp gas tank with fresh fuel.

    Bob
    Member - WAFNC
    1997 22 Classic (sold)
    1997 Formula 271 Fastech
    502 Mag MPI

  9. #9
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    So today I got smart and pulled the MEFI codes using the OBD port on top of the motor. I pulled two codes 14 and 34. 14 - coolant sensor voltage high (low) 34 - Mass Absolute Pressure Sensor low. Can he MAP sensor be pulled wo removing the intake manifold?

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    I have the same issue can I scan it with a regular OBD II scanner?

  11. #11
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    I had a similar problem several years back. Turned out to be a faulty fuse in the ECU (?) box. It wasn't blown but must have had a hair line crack in the element that would intermittently separate and then remake contact. Not sure which one it was but replaced all of them and the problem hasn't recurred since.
    RedDog

    President - West Vestal Donzi Owner's Group
    Knoxville, TN

  12. #12
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    Quote Originally Posted by RedDog View Post
    I had a similar problem several years back. Turned out to be a faulty fuse in the ECU (?) box. It wasn't blown but must have had a hair line crack in the element that would intermittently separate and then remake contact. Not sure which one it was but replaced all of them and the problem hasn't recurred since.
    I'm referring to the blade type fuses in the box mounted between the exhaust manifold and the intake
    RedDog

    President - West Vestal Donzi Owner's Group
    Knoxville, TN

  13. #13
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    Quote Originally Posted by brwn234 View Post
    So today I got smart and pulled the MEFI codes using the OBD port on top of the motor. I pulled two codes 14 and 34. 14 - coolant sensor voltage high (low) 34 - Mass Absolute Pressure Sensor low. Can he MAP sensor be pulled wo removing the intake manifold?
    Neither of these will stop the engine from starting, and just because a code was set, that doesn't mean that the sensor is bad. Don't chase sensors. Get a manual and start going through the troubleshooting section, 5C.

    B
    Member - WAFNC
    1997 22 Classic (sold)
    1997 Formula 271 Fastech
    502 Mag MPI

  14. #14
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    Quote Originally Posted by brwn234 View Post
    I've got a 2000 7.4 MPI that won't start. I'm leaning toward a fuel issue. Here's what's going on:

    Uncovered it from the shrink wrap earlier this spring. Put it on the hose and could barely keep it running and when it did the rpm would rise and fall like it had a big cam. I heard the IAC opening and closing like crazy so I thought that may not be operating properly. Frantically threw a new IAC in it trying to get on the water. Ran a bit rough, but ran. Got to to the lake and nothing.

    If I cycle the fuel pump by turning the ignition on and off several times without bumping the starter it will fire for a brief moment. The pump is definitely spinning but at what pressure I dunno. I've checked simple things like my Racer fuel filter, drained and topped with fresh fuel. Replaced the worn vacuum line on the fuel regulator. Previously when I'd open the fuel filter id need to purge any air from the lines using the port just behind the thermostat housing. There's no air in the lines, I'm pretty sure.

    Just today I bought a fuel pressure test kit. it was kinda late so I couldn't start it up but turned the ignition switch on to cycle the fuel pump. 35-45 psi several times. Where do I go next? I was almost completely convinced it's a fuel issue but not so much anymore. Tomorrow I'll attempt to start it again while monitoring the pressure. How can I determine if it's a pump vs a regulator issue?

    Thanks
    What was the out come?

  15. #15
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    I haven't had much time to work on it lately. I'm currently getting ready to move and have a bunch of other stuff goin on. This is where I'm at now;

    I removed the MAP sensor, cleaned it off and reinstalled it. the boat will run at idle now but very rough and will die with any throttle. The fuel pressure will jump up to 80psi when the pump is priming and then rest at 35 when it stops. when running the pressure bounces around 80.

    This pressure when the pump is running seems very high. Could this be due to a regulator issue or possibly a partial line blockage?

    Oh yeah, reset the codes and they do not come back.

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