Hot Engine stumbles, hesitates, and lacks power.
 

I have a 2001 Miriah Z275 Shabah with a 454 Mag MPI Generation VI.

When hot, the engine stumbles, hesitates, and lacks power above 2,000 RPMs. Runs like a champ when cold. Very frustrating.

Please don’t ask if I checked the fuel, filter, fuel pressures or any of that. This problem is electrical.

If anyone has experienced this and found the fix I would appreciate hearing from you.

PROBLEM WAS SOLVED AT ONE TIME:
After replacing the Ignition Control Module (ICM) or sometimes called the Ignition Module and the pick-up coil in the distributor with new Sierra Marine parts and the problem was resolved (for a while). Now it is back and I am making the assumption that the ICM is again bad from overheating. I have ordered another and will update after the fix.

ATTEMPTS MADE TO FIX ISSUE:
To get to this point I started out replacing parts one at a time. Here is the order, list and effect:

1. Replaced the oil sending unit (my oil pressure gauge was intermittent and this resolved the gauge issue but did not affect engine performance).

2. Replaced the ignition coil. This improved the performance slightly when cold, but did not resolve the problem when the engine was hot.
3. Replaced the spark plugs and plug wires. Slight improvement when cold but no change to the hot engine issue.

4. Replaced the ICM and pick up coil in the distributor. Re-timed the engine. This fixed the problem for about 4 hours of run time. Now it is back to doing the same thing. From what I have read this is a fairly common issue to have the ICM overheat and cause the engine issues when hot. Still runs great until the engine warms up.

5. Replaced the Intake Air Sensor and the Manifold Air Pressure Sensor. (No change).

6. Replaced the Throttle Position Sensor. (No change.)

NEXT STEPS:

I ordered a TechMate Pro scan tool and plan to run it at the dock and on the water to see what codes are set when the problem exists.

I also ordered another Sierra Marine ICM and pick up coil. I plan to change the ICM first, and yes I used lots of heat sink compound and will again.

I am also going to look at any grounding issue, as I have read a poor ground can cause the ICM to burn up. The battery cables are grounded well and I am not really sure where to look for other grounding issues.

I am considering adding a separate bilge blower system to aim at the distributor area to run all of the time (separate switch) and leave on when we stop the boat for a few minutes. My thought is that the engine heat is building up more when we are parked swimming, etc. There cannot be much air moving in the engine compartment parked or running and I think this contributes to the problem of burning up the ICM’s.

Re: Hot Engine stumbles, hesitates, and lacks power.
 

The engine ground is located on the rear of the engine (tough to see). There is a large black battery cable that is mounted to a stud where the engine and bell housing meet. I doubt this is your issue as you have not complained about a starting issue.



I believe the ground for the ignition module is through the bolts that hold the module to the body of the distributor, through the distributor, through the engine block and back to the battery negative post. If this path has excess corrosion or even a paint build up you could have a compromised ground. I would check it carefully and clean up all the mating surfaces with a wire wheel and razor blade to ensure a good clean ground path.

Have you checked the voltage coming out of your alternator. If the voltage is excessively high you could be burning up ignition coils and modules.

Good luck.

Re: Hot Engine stumbles, hesitates, and lacks power.
 

Hi Steve. Thanks for the input. I will look for the ground on the back of engine and make sure connection is clean. Does that ground go to the engine start battery or the other two batteries?

I will also make sure all the distributor parts are corrosion free and grounding through the system.

I will also check the alternator voltage output. I have noticed that when we activate both trim tabs the electrical load slows down the engine rpm - even at full throttle. Not sure if this is normal.