Hello

I had some issues with my 496 a couple of months ago with strange engine RPM that lived its own life, turned out to be the TPS sensor that was bad. Switched this and the problem whas gone.

But I still have some small issues with the engine RPM. The engine runs porely between 800-1500 rpm when I give the engine a little bit of gas (when not on plane), just cruising around.
It sounds like one cylinder is missfiring or something. But when im running the boat on plane/full gas its fine, no problems..

What can cause this to happen? Bad spark plug wirings?
I run the OEM Aldelco spark plugs.

Thanks for help where to start searching

Plug or plug wire.. Get a spark tester with the clear window where you can see the spark.

Chris

coil misfire possibly, you really need to scan the PCM and see if its throwing any codes

I bought a spark tester, gonna try that one on all the plugs.

I also gonna test to measure the cables.
What resistance should the cables have?

Okey thank you, I dont have any Scan tool, just the Mobile VesselView ”Fault codes scanner”.
Have seen codes before on the VesselView but that was the code for bad TPS.
No codes from the VesselView after I switched the tps what I could see.

Whit coil misfire, could it be different things related to this ?

Found this on another forum:

Final Diagnosis: Bad plug wire <br /><br />I know, sounds ridiculous. The 496 is a DIS (Distributorless Ignition System) engine, meaning that it has a coil for each cylinder. Four on each side of the engine, mounted on the valve cover.<br /><br />In this case, the #7 cylinder spark plug wire was leaking secondary ignition voltage at the coil tower, arcing to the coil frame. The coil frame and entire valve cover became part of the secondary ignition system. Due to a rubber gasket and appearently pretty decent paint job, the valve cover was not grounded and the ignition voltage found the easiest path to ground (intermittently) was arcing across a gap from a bare spot (chip) in the paint on the valve cover next to a mounting bolt, to a bare spot on the upper part of that mounting bolt. Just so happens that mounting bolt was directly under the ECM . The 40,000 volt discharge (about 2.5 inches away) was creating enough EMI (Electro Magnetic Interference) to momentarily shut down the ECM.<br /><br />I only found the problem because I happen to be at just the right place to actually see the spark under a wire bundle. You could not hear it due to normal engine noise. I pulled the wires away a little and I could hardly believe it, a nice fat blue spark from the valve cover to the mounting bolt. Directly under the ECM . The engine lurch happened at the exactly same times as the spark occurred. I put a jumper wire from the valve cover to a good ground. Problem went away. Took it off problem came back. Still unsure what cylinder it was, I put it on the scope. Found an intermittent misfire on #7. Visual inspection confirmed arcing from the coil tower (under the plug wire boot) to the coil frame. Changed #7 coil, wire and plug. No more problem. <br /><br />Thanks for your responses. Maybe someday this will help someone with a similar problem.

If looking for wires, FOund this option the other week, compare to the $500-$600 factory merc wires

https://sparkplugwires.com/scott-mer...plug-wire-set/

Thank you. Also found these:
https://www.marineengineparts.com/cr...-plug-wire-set

Dont know if they are as good as factory Merc wires

Those are the same as merc oe. Atleast they where up to last year when I linked those and some others here the other year. Do a search under my name and those wires.

Possibly Crankshaft Position Sensor. They can go bad without any codes. Do you have any beeping from the engine's alarm? IAC going/gone bad can cause issues at lower RPM ranges too.
Try running the engine the dark, to see if you can see plug wires arcing anywhere. Replacing spark plugs is never a bad idea. And when you do inspect plug wires and ensure they are all snapped on correctly.

Or some Moroso 8mm wires for $70. I ran a set this summer, worked good and fit right.

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