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Mercruiser 5.7L dies at about 3/4 throttle.

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  • Mercruiser 5.7L dies at about 3/4 throttle.

    Issue 1: Tach not working stays at zero. Tach signal wire is conneted to negative terminal of coil. Assuming the sensor inside the distrubutor is good if i replaced the ignition coil would this fix the TACH also assuming the gauge is good. I tested the tach signal wire and it is not shorted to ground.

    Issue 2: Engine cuts out at about 3/4 throttle if i do not throttle back when it starts to cut out it backfires/misfires. If i back off the throttle quickly when it starts to cut out it putters for a few seconds sometimes recovers but mostly dies. A rich smell of fuel is present and it is hard to start after this happens. The longer the engine runs throughout the day the worse it gets. By the end of about 4-6 hrs of run time it will sometimes just cut off when idling. Trouble getting started by the end of a day of boating. Next day fires right up first time.

    I am leaning towards a timing issue/bad coil but i am not sure. I have checked the fuel bowl immediately after the engine dies a few times and there was plenty of fuel in the fuel bowl(trying to eliminate a fuel delivery issue).


  • #2
    Re: Mercruiser 5.7L dies at about 3/4 throttle.

    Please post some more information - what year and model engine? Serial # would help as well so the experts can better diagnose the problem.
    Champ II - 1986 Formula 206 Spider, Vortec 383 Stroker/Alpha One (until it blows up, then maybe a Bravo)

    My 383 Stroker Project:
    http://forums.iboats.com/showthread....55#post3554555

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    • #3
      Re: Mercruiser 5.7L dies at about 3/4 throttle.

      I would try it with the tach unhooked at the coil.
      rebuilding 3 classics
      17 Irvette I/B CC 99%
      20 Shamrock I/B Pilot house. 50%
      28 Slickcraft I/B FB 60%

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      • #4
        Re: Mercruiser 5.7L dies at about 3/4 throttle.

        Originally posted by thorton32 View Post
        Issue 1: Tach not working stays at zero. Tach signal wire is conneted to negative terminal of coil. Assuming the sensor inside the distrubutor is good if i replaced the ignition coil would this fix the TACH also assuming the gauge is good. I tested the tach signal wire and it is not shorted to ground.

        Issue 2: Engine cuts out at about 3/4 throttle if i do not throttle back when it starts to cut out it backfires/misfires. If i back off the throttle quickly when it starts to cut out it putters for a few seconds sometimes recovers but mostly dies. A rich smell of fuel is present and it is hard to start after this happens. The longer the engine runs throughout the day the worse it gets. By the end of about 4-6 hrs of run time it will sometimes just cut off when idling. Trouble getting started by the end of a day of boating. Next day fires right up first time.

        I am leaning towards a timing issue/bad coil but i am not sure. I have checked the fuel bowl immediately after the engine dies a few times and there was plenty of fuel in the fuel bowl(trying to eliminate a fuel delivery issue).
        Ayuh,... Welcome Aboard,....

        Issue #1, Replace the Tach, not the coil....

        Issue #2, is a Fuel Delivery issue, start with checking the contents of the Filters....
        Any Grease is Better,..... Than No Grease at All.......

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        • #5
          Re: Mercruiser 5.7L dies at about 3/4 throttle.

          S/n om 317-516 2003 merc 5.7 2bbl carb thunder bolt v ign ski centurion
          Going to test sensor inside distributor via spark gAp tester to eliminate that as a issue can not find definative way to test ign coil
          The tach signal I believe is a magnetic pulse is there a way go test it
          Is it a 4-20ma signal
          Need a way to test tach gauge or signal
          Going to install inline fuel gauge maybe I can eliminate that
          Still leaning towards timing issue
          I rebuilt the carb
          Before rebuilding carb I could only run the boat at cruising 5-10mph
          Also replaced plugs for first time since 2003

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          • #6
            Re: Mercruiser 5.7L dies at about 3/4 throttle.

            if it is a timing issue why not just set the timing to rule it out??

            if you have spark then the coil is okay, the sensors usually just fail altogether, rarely is it an intermittent issue or issue at certain rpm - if it is the original sensor i would recommend replacing it though, the latest style has zero issues

            have you checked fuel pump pressure??

            check the tach wiring to be sure it has proper voltage and ground and the sender wire is connected properly and intact
            Sometimes when people try to save a penny, they end up spending a dollar

            It is best to fully understand how to properly operate, inspect, troubleshoot and repair your equipment

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            • #7
              Re: Mercruiser 5.7L dies at about 3/4 throttle.

              Problem solved except for the Tach. Tested Coil with a spark gap tester it was good but i replaced it anyways because it was old and had a lot of corrosion on the contacts. After replacing the coil and knowing that it was good I tested the Ign. Sensor inside the Distributor. To do this I removed the high tension lead from the Distributor and installed a spark gap tester then grounded it. Then unhooked the White/Green wire that connects the Ign. sensor to the Ign control module. Next i made up a wire with male bullet connectors on each end long enough to reach a good engine ground. Hooked one of the ends of the male bullet into the ign control module side of the wht/green wire. With the ignition switch in the Run Position i hit the male bullet connector from the wht/green wire hooked to the ign module side of the White/green wire to a good engine ground 2-3 times per second to simulate a running engine. There was spark initiated which meant that my ign sensor was bad. When i went to install the new ignition sensor my distributor contacts were badly corroded so i replaced dist. cover/sensor wheel and rotor. i was hoping a new ign sensor/coil would fix my tach but no luck on that. i am convinced now that my tach gauge is bad. The reason that i could only go 3/4 throttle was because when the ign sensor is bad the ECM swithces into a base timing mode and the engine output is lowered. After replacing the ign sensor and tuning up my distributor My boat ran great. Went full throttle off and on for about 20 miles no issues. It was not a fuel issue as I suspected. As for my tach i am sending it into Faria for repair it will end up costing me 80$, but it is cheaper than buying a new one which is about 200$.

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              • #8
                Re: Mercruiser 5.7L dies at about 3/4 throttle.

                Thanks for actually posting your result (unlike most). Sounds reasonable, happy boating!

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                • #9
                  Re: Mercruiser 5.7L dies at about 3/4 throttle.

                  Yes, thanks for letting us know. I have a similar problem and it gives me a place to start.

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