Mercruiser 350 MAG MPI missing only at 3000 rpms

[kevinwatkins2003]

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

 

[Maverick7]

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

Mercruiser 4.3L - Stumbles at 3000RPM

2004 Mercruiser 4.3L Thunderbolt V Alpha 1 Gen 2 Engine S/N: 0m670495 Crownline 180BR I took the boat out of storage today. It ran perfectly fine for about 30 minutes at varying rpms along our ride. After the first stop, we took off again and I normally like to cruise at about 3000rpm/22mph...

forums.iboats.com

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.

 

[kevinwatkins2003]

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.

 

[muc]

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

 

[kevinwatkins2003]

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.

 

[cyclops222]

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