Misfire under load

[mfehnel]

Put the boat in the water today and the boat misfires as soon as I go over 3200 rpm. Put a timing light on it and notice the light cut out every once an while in gear are putting along at 1000 rpm. After noticing this I put the light on other plugs and notice they were cutting out as well. I sent the EMC to marine power and they stated knock sensor replaced it and the engine still misfires. Would this maybe a ignition module?

[Thegerman618]

Year/model

Check your fuel water separator and high pressure fuel pump.

Try and diacom if possible at your dealer.

[JCP]

When was the last tune up? Cap, rotor, plugs, plug wires?

[chpthril]

In most cases, that fish bite stutter type miss under load is ignition secondary, as in spark to the cylinders. Cap, rotor, wires and plugs first. Next most logical is the ignition module.

[freeheel4life]

Agree with Mike. If you cant nail it down to an ignition issue the next is injectors. Have had dirty injectors cause this problem as well. Cylinder cutout test can help confirm.

[boatwakes]

Engine model, year and HP please if it still occurs after basic tune up.

[Thegerman618]

Marine Power, most common would be MP 5.7 340HP?

I noticed this problem with mine last trip as well, but I am due for a fuel filter change so I am gonna start there.

[ktm450exc]

I had the same problem. I replaced cap, rotor and plugs and my problem was gone.

[mfehnel]

So the engine is a MP 06 5.7. 340hp on 24ve. Replaced the injectors and fuel pump as well the wires, plugs, and cap.

[freeheel4life]

Hook up a fuel pressure gauge and run it while watching and verify fuel pressure at WOT under load. Ignition module would be other possible culprit judging from the other parts you've thrown at it. Fuel filter could still be cause, which is why I say hook up to Schrader at fuel rail while running under load.
Worst case scenario, check compression. You are starting to run out of boxes to check in the diagnosis process but not quite exhausted...yet

[IDBoating]

Not sure where this function is located in your engine, but... I've seen similar behavior twice. Once it was a failing Rev Limiter (which on that engine was a separate module) and the other time it was an intermittent connection on the killswitch circuit. The first one was easy to diagnose - temporarily disconnect the limiter and the problem went away - but the second required me to put a scope on the signal line. The movement of the boat caused a flaky factory crimp splice to make-break, which killed spark very briefly and made it feel like the engine was misfiring because it kept running when the connection wiggled back into continuity again.

I HATE crimp splices. Solder+heat shrink for me, every time. Crimp splices are STUPID in a high vibration environment, almost designed to fail. [/rant]

Anyway, I don't know if/where you have a rev limiter but I'd check that, and the killswitch circuit. Could be something really simple and easy to fix.