Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

[phylly123]

2005 Searay Sundeck 240, Mercruiser MCM 5.0 L MPI

While operating my boat yeaterday I had the motor at 4100 rmp's. The rpm's increased to 4600 without touching the throttle. After about 20 seconds they decreased to 4100 then back upto 4600. The rpm's stayed at 4600 until I backed down on the throttle. I heard that this may be due to the ECM but don't know enough about the engine to really be able to diagnose anything. The manufactures WOT is 5000 but I can't seem to ever reach that. Is there something wrong the ECM? Engine runs great, starts right up, idle's smooth. Anyone have any suggestions? Any help would be appreciated.

 

[trashman83]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

I have a 1994 Mercruiser 5.0 2 0 carb. I have the same problem with the RPM increasing and decreasing. Did you figure out what was casuing it.

Thanks- Trashman83

 

[achris]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

Check/Change spark plugs... Then suspect Idle Air Control valve.

Chris...

 

[phylly123]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

I have a 1994 Mercruiser 5.0 2 0 carb. I have the same problem with the RPM increasing and decreasing. Did you figure out what was casuing it.

Thanks- Trashman83

No not yet

 

[phylly123]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

OK thanks i'll try them both

 

[trashman83]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

My mechanic said it sounds more like an electrical issue rather than a fuel issue. When the RPM?s change it?s very sudden like a switch being thrown. I changed the wires and cap since the originals had never been replaced.


Thanks ? Trashman83

 

[sethjon]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

2005 Searay Sundeck 240, Mercruiser MCM 5.0 L MPI

While operating my boat yeaterday I had the motor at 4100 rmp's. The rpm's increased to 4600 without touching the throttle. After about 20 seconds they decreased to 4100 then back upto 4600. The rpm's stayed at 4600 until I backed down on the throttle. I heard that this may be due to the ECM but don't know enough about the engine to really be able to diagnose anything. The manufactures WOT is 5000 but I can't seem to ever reach that. Is there something wrong the ECM? Engine runs great, starts right up, idle's smooth. Anyone have any suggestions? Any help would be appreciated.

When the RPM rose, did the MPH also rise? If not, may be a prop issue.

 

[trashman83]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

When the RPM rose, did the MPH also rise? If not, may be a prop issue.

The miles per hour increase with the RPM. It sounds just like yours. It cranks the rpm and mph runs about 15 - 20 seconds and drops back down. The higher the speed the more it jumps.

trashman83

 

[achris]

Re: Mercruiser MCM 5.0 L MPI RPM's increase and decrease without moving throttle

Trashman. Please start your own thread... You may have a problem with the same symptom, but your fault might be completely different to the OPs (especially with MPI engines)... It's also very confusing for us trying to offer suggestions when you are walking all over the original post... So, start your own and we will deal with each person's problem separately... Thank you...