Mercruiser 350 MAG MPI missing only at 3000 rpms

2009 Harris Grand Mariner 250 SEL Tritoon
Mercruiser 350 MAG MPI Bravo III
serial: 1A335811
300 hours

Engine misses/hiccups only in a certain RPM range around 3000 to 3500. At idle and higher throttle it seems to run fine. It only misses or hiccups in that middle power range on throttling up and when you throttle back down to the range. It is when you are getting on plane and coming back off plane. If you hold at a constant throttle in that range, it does seem to surge on rpms. Seems like im really hearing it out of the rear of boat when it missesz like the exhaust seems louder. I've had the same boat before and same setup and exhaust sounds different. No captains call.

I do know the engine possibly sit in water for quite awhile, as I see rust water lines up fairly high on the exhaust manifolds.

Local shop has had a computer on it out on the lake and ran it. Haven't been able to diagnose.

Compression is good.
Plugs/wires changed.
Fluids changed.
Not sure age of fuel, but did add 28 gallons of non ethanol to top it off and ran 5 times since with no change.
tried a new crank sensor
Checking to see if a new distributor cap was tried.
Made sure plug wires were not arcing on block

I just dealt with a similar issue, not sure if it’s the same case as yours but worth a look.


Thought it was a timing sensor assembly, turned out to be wires coming out of alternator shorting out on the manifold. Take a look at the bottom of that post for pics.

Thank you. I will check this out. Funny thing is I did have a new alternator put on. I bought the boat and had delivered. Never ran it personally before i had it serviced. Took it to shop and they made all the repairs above. So I can't compare before and after.

Post #18 has a link to a Service Bulletin (thanks @alldodge)
take a look at pages 12 and 13

Update: I ran boat at night. Did not see any wires arcing against block. I did have the old 50amp fuse with yellow flag, I bypassed that and nothing changed.

It now seems like I have an intermittent high pitched slapping/firecracker popping even at very high rpms. It reminds me of the sound my 1990 GMC Jimmy 350 TBI made when it had water in the fuel and used HEAT in fuel tank to clear up. Friend said it may be distributor going in/out? Of course I still have the mis fire at the low rpm range before I get onto plane. You can feel it more than you hear it. I only hear it coming out of the exhaust in the water, don't hear it at engine.

I never buy anything without ME testing it thoroughly. Always.

muc said:

Mercruiser 5.0 MPI #3 cylinder ignition miss

Hello, My Rinda indicates #3 cylinder misfire (2011 5.0 MPI with 255 hrs). I get about 100 misses in 3 minute? No code but I can feel it. I checked plug wire for resistance it appeared good. I will order a new plug, but my question is what is the best distributor cap to buy. I hear the...

forums.iboats.com

Post #18 has a link to a Service Bulletin (thanks @alldodge)
take a look at pages 12 and 13

Click to expand...

Unfortunately this is not my issue after testing.

Updates:

If I shower down quickly I can essentially get past the engine missing. It still only occurs under normal throttle up to get on plane in the lower rpm ranges. Then once you power back down off plane it occurs again in same rpm range. So it seems to me it's not really associated to the engine being under its heaviest load.

Wires, plugs, cap changed
Used lap top and dropped out each cylinder and still did it dropping each.
Crank sensor
Ran in dark and looked for arcing of wires
Powered 12 clean volts with jump with to ignition coil
New distributor
New coil and module

Mechanic thinks next thing may be cleaning fuel injectors? But just seems Id have trouble more that at a certain rpm band? Hate to spend that money. Past that, he is stumped and seems to think I should wait until something else gets worse or breaks.

Overall it runs fine at idle, just past idle and mid to top rpm. Top speed is right where it should be.

Did the new distributor come with a new rotor?
From past experience when a rotor heats up they will go to ground at coming on plane and coming off as that is when he most load is on the ignition system.
I once had a similar problem with the same engine in my Donzi. I finally cured it with the most expensive iridium spark plugs made for that engine.
I realize the plugs were already changed but which one were installed?