I’m really scratching my head here so I’m posting this to see if anyone else has had this issue and hopefully to learn us all somethin in the process. 2006 525 efi 396hrs. Whipple and heads done about 75hrs ago. Bought boat, fired up in driveway multiple times before going out on the water, ran great, have videos to prove it. First time out, second time getting to cruising speed all the sudden I hear solid alarm for maybe 1 second then port motor shuts down at about maybe 3000rpm, not fast at all. Since then motor starts hard, runs crappy etc, narrowed it down to no spark on cylinder #4. I’ve swapped plugs, wires, coil packs, and even computers with stbd motor. I didn’t try to run stbd ecm on port motor though, only put port ecm on stbd motor and it ran perfect. Have not checked compression but there is clearly no spark with spark checker. Scan tool shows no issues, cleared old faults. I did notice timing advance on port motor is all over from 10-38deg, stbd is 8-13, but I’m assuming that’s port ecm trying to compensate idle for #4 doing nothing. Any ideas?

Bad spot in wiring harness from ECM to coil?

That’s my next check, pin out from ecm to coils. But...#7 shouldn’t be firing either then, but it is

Valve could be sticking? Broken valve spring?

Tim

Could be, but would that cause no spark on that cylinder?

Excuse me if I missed it, did you pull the plug , take a good look for damaged or missing ground strap ir electrode and change it anyway if looked okay ?An inline spark tester won’t light with a bad plug either.

Yea I’ve swapped plugs, even tried a brand new plug.

If you are getting a misfire code on the ecu yet it works at idle and other rpm's but misfires randomly then yes it can be a valve sticking or broken spring, if the valve is not sealing the combustion chamber it is a misfire. I believe on your engine you have more then one plug getting fed per coil. If that is the case and you are moving coils around, plugs, plug wires then the problem is in the cylinder itself, I feel

Tim

Could the crank sensor be the issue. I know seems odd only #4 would not fire

Yea, with the wasted spark system, if #7 is firing #4 should be as well right?

But the plot thickens. No compression on #6 cylinder. Hoping this has something to do with no spark on #4. Bent valve... When it rains it pours I guess