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mercruiser 7.4mpi has sudden loss of rpm-out of ideas

S

Scubadave16

New member
I have a 2001 Mercruiser 7.4 MPI engine in a 29 foot boat. this spring it started and ran just fine. With WOT I got to 4100-4200 RPM and the boat got up on plane. While cruising, the RPM dropped and i lost plane all of a sudden, then the RPM briefly surged back up. This repeated several times and since then I can only get 3600 RPM and cannot get the boat on plane. The engine runs very smoothly at idle and even when struggling to get on plane there is no hesitation, knock, surging or anything, just limited RPM. I had some fuel remaining from last year, but had put marine STABIL in it. I had also added 30 or so gallons at the start of the season. I added Lucas fuel injector cleaner. I have changed spark plugs, distributor cap, rotor, spark plug wires, and the fuel/water filter. I have cleaned the flame arrestor. I put a fuel pressure guage on the fuel rail and got consistent 38-40 psi when at idle or at WOT. I had a mechanic try to see if any codes were stored. there were none. The mechanic swapped out the spark/timing advance module with no change. Compression in the cylinders ranges from 135-150. THe mechanic checked and the throttle is wide open when the handle is fully depressed so no binding of the cabled. The outdrive is all the way down. the propeller spins freely and the bottom is clear of marine growth. both the trim sensors on either side of the gimbal housing are bad, but they have been bad since i bought the boat 5 years ago. The serial numbers for the engine have been worn completely off the tag so i cannot provide that information.

I am out of ideas and the boat is pretty much useless as i can only idle along at 8mph or 15mph at WOT.

any help or ideas would be appreciated.
 
fuel pressure might be ok but what about fuel column? put a laser on the fuel temp. just got in a CFIII unit that a shop replaced and the only thing that was the reason was that the cooling system for the pump was clogged up with sand and debris
 
Thanks for the Input. I assume u r talking about the cool fuel module. It is unlikely that this is clogged as I only run the boat in open fresh water usually 10-30 feet deep. Not having a laser is there some other way to test this? Also when it first occurred the water was probably only 40F. I understand this module is very difficult to even access let alone replace
 
Your fuel pressure is a tad low but your gauge might be a bit off. Pressure should be 43 at WOT or at idle with vacuum line removed from Reg.

Compression in the cylinders ranges from 135-150.
Click to expand...
Compression numbers are exactly 10% apart. Was this done warm or cold?

Check to see if the distributor is advancing
 
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