RPM and Fuel Flow Surging through power band on 7.4 MPI

Chicago Sea Ray

Chicago Sea Ray

Member
Mar 21, 2016
57
Chicago
Boat Info
1997 Sea Ray 370 Express Cruiser
Engines
Twin 7.4L Mercruiser MPI
I am hoping someone with much more experienced than I can help diagnose a problem I'm having in my port motor... a 1997 Mercruiser 7.4 MPI.

Engine is generally running very well... starts right up... idles smoothly at 600 RPM... consistent throttle response through the power band... etc.

The issue I'm having is that at 1200 through 4200 RPM, there is a sporadic and inconsistent RPM and Fuel Flow surge on occasion. I can run for 10-15 minutes with no sign of this issue... and then all of a sudden the RPM can jump or dip between 50 and 200 RPM with the fuel flow increasing. The surge lasts from 1 second to 15 seconds and then corrects back to where it was supposed to be. This can happen several times in a row or only once or twice during a quick ride.

Any thoughts on how to solve for this issue? I've checked the TPS sensor for variations during this event... no change. I've checked the fuel... not contaminated. The fuel pump is 2 years old so don't expect that is the issue.

Appreciate any and all thoughts/guidance!
 
watch the amp gauge next time it happens....it may be possible there is an issue with the alternator or voltage regulator causing the alternator to place an unusually high drag on the engine off and on....

cliff
 
The plugs are within the year and the wires in the last two. The injectors have not been changed during my ownership. May check to see what those cost and if a swap of them all makes sense for good measure.

To be a bit clearer, the ECM reads a heightened fuel flow when the surge is happening. An example: running at 3000 rpm, can surge to 2900 and jump from 14.3 g/hr to 16.5 g/hr.

I will check the alternator next time as well... but can it really create that kind of drag on a 454 spinning at 3200 rpm that I or it would even notice?

Thank you very much to all for the continued feedback! Extremely helpful.
 
{I will check the alternator next time as well... but can it really create that kind of drag on a 454 spinning at 3200 rpm that I or it would even notice?}


I'm thinking that it may be possible that your batteries are a little weak which would force the voltage regulator to call for extra charging from the alternator.....this additional charging mode cycling on and off could be enough to notice in engine rpm's....failing bearings in an alternator can also increase drag but that would be a constant drag and not fluctuations like you are seeing....

good luck....
cliff
 
CliffA said:
{I will check the alternator next time as well... but can it really create that kind of drag on a 454 spinning at 3200 rpm that I or it would even notice?}


I'm thinking that it may be possible that your batteries are a little weak which would force the voltage regulator to call for extra charging from the alternator.....this additional charging mode cycling on and off could be enough to notice in engine rpm's....failing bearings in an alternator can also increase drag but that would be a constant drag and not fluctuations like you are seeing....

good luck....
cliff
Click to expand...

Cliff,
Thanks for the additional details! The batteries are brand new as of last year... two per engine and identical across both Port and Starboard. But, I will absolutely watch voltage fluctuations on the next run and see if there's something funny going on when the surge occurs.

Thanks again
Nick
 
Update... because the RPMs would drop and fuel consumption would increase at the same time... it was determined to be an ignition issue. I replaced the plugs, wires and distributor within the last year... so that left the ignition coil. I swapped out the ignition coil for a new one and we’re running perfectly now :)
 
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