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Mastercraft X2 2005 MCX 350 Misfire when warm

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  • Mastercraft X2 2005 MCX 350 Misfire when warm

    Hi - I'm hoping someone can shed some light on an engine issue I seem to be having with my 2005 Mastercraft X2 MCX 350 hp V8.

    When starting from cold the engine runs perfectly well. After approximately 20 minutes (when the engine is fully warmed up the engine starts to misfire but in quite a dramatic fashion. The engine sounds like it's missing on about 4 or 5 cyclinders, so not the usual sound of just one cyclinder missing - this is quite an alarming breakdown of something.

    If we stop to idle an leave it to cool down for a moment, or even if we open the engine cover then the boat will do another 5 minutes of perfect running until it decides to shudder and misfire.

    I've done the following to try and diagnose certain aspects of the ignition but I'm no mechanic:

    1) Set the engine into a quick neutral idle and pull each injector connector off. Result is the engine revs slow which you would expect when losing one injector. This happens for all injectors so they seem fine.
    2) Set the engine to a quick neutral idle and pull each plug lead. Again engine revs dip and this returns when the plug lead is re-attached. This happens for all 8 plugs. So the plugs seem fine.
    3) If we run the boat with the engine cover held up to allow cool air into the engine bay then the issue does not occur.

    So I'm assuming something is getting hot/warm and causing the issue - but I've no idea what and I don't want to start replacing expensive engine parts for the fun of it.

    Anyone come across this before?



  • #2
    My first thought is what does your cap and rotor look like?


    • #3
      Cap and rotor are brand new. Plugs new at the end of last season so have probably seen about 5 hours use.


      • #4
        Check engine light? Any codes stored?

        Making boomers great again!! Boomin'


        • #5
          only when hot then it might be the temp sending unit... search on here there are a couple of threads on it... a $20 part.. works for most but not all...

          otherwise if it has run low on fuel in the past.. the next culprit is the fuel pump to check


          • #6
            May not be heat, may be lack of oxygen. If it doesn't do it with the cover open I would start by looking for an exhaust leak. Opening the hatch would not dissipate enough heat to instantly fix the problem when running. It will however pump a bunch of fresh air I. And allow the motor to breath properly.
            1986 Marlin Magnum Skiier-sold
            1999 Maristar 210VRS-weekend ride
            1996 205 with LT1 cracked block-stitched back together


            • #7
              Hi all - I'm back. So continuing with the saga of my engine running rough when warm..

              I have got a little further with this issue as I've replaced quite a few parts now - coil, thermostat, ECM engine temp sensor, rotor arm, distributor cap, plugs, leads.

              I'm now looking at replacing the Ignition Module that is housed in the distributor. I can't seem to source one in the UK and I'm hoping someone will be able to suggest where I can get one in the states and have it shipped to the UK.

              The only code I can get off it is 849 4H05.

              The engine is an Indmar MCX350 5.7 V8.



              • #8

                Well it's finally done.

                The boat is back to running as it should - granted I've replaced a few bits along the way but the boat is 12 years old and it won't harm at all having those new bits.

                So the issue was the ignition module in the distributor. I replaced it with


                All's well that ends well.