7.4 MPI losing RPM's.....

roosevelt14

roosevelt14

Well-Known Member
Apr 15, 2007
597
Western Lake Superior
Boat Info
2002 450 Express Bridge
Engines
Cummins 6CTA 430hp ZF280IV
My port engine (1996 MPI 7.4 mercruiser) is losimg rpm's...Motor runs great......but when running 3600-3800 RPM (this just happens to be where I run my engines) my port engine will start to drop rpm's, till it reaches 3400 rpm and it will stay there for a minute of two and then go back to the RPM's that I had it set at?? It will then stay there again for a while may be 5-10 minutes and then starts all over again losing rpm's and goes back to 3400....over a 3 hour trip this happened about a dozen times....

I have replaced plugs wires cap rotor....Motor sounds great when it does lose RPM'S...If I try to advance the throttle when its at 3400 nothing happens..no faster no slower??:huh:
 
Fuel flow problems? Did you check and replace the fuel filter?
 
Are you losing speed when the rpm's drop?
 
yes I replaced the fuel filters.....Tomorrow I'm hooking up a presure gauge to the fuel rail and going out and run it to see if it holds when I start losing rpm's
 
Good plan checking fuel rail pressure. The downside is that if it is the regulator, it isn't on top of the engine where GM intended. Mercruiser puts it down on the bottom with the fuel pumps so that it is nice and inaccessible.

Best regards,
Frank
 
If you don't find it, you might want to try a scanner, such as Rinda.

If you get a scanner, contact Merc for the pertaining manual.
 
I have had a scanner hocked up, a Mercruiser Scanner which I think is made by Rinda...Any ways there were no code's.....
 
The merc scanner is made by Rinda. I have one and it's a decent tool. Didn't suggest scanning because it doesn't sound like a problem that would turn a code. Suggesting scanning would have just driven you crazy to find one if you didn't have access to the tool or would have been redundant if you did have one since, as you said, you would have already scanned.
 
I had a very similar situation with my '93 7.4s in my 330DA. Couldn't get the darn things to sync up either....turned out to be the coil.
 
Finally got back on the water, with fuel rail gauge installed...she held right at about 32-34 psi the whole time, I'm still losing RPM's....

My friend who was watching the gauge said it all most sounded like it was fluddering when we were losing RPM'S:huh:
 
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Im having same problem please let me know what u find out.
 
I was thinking of the scanner to monitor while you are experiencing the problem. But on second thought, without being smartcraft, it may not tell you much.
 
Just a thought here. I don't know if your engines have a Mass Air Flow Sensor on them or not. If so: Perhaps something could be on one of the sensor screens. I've experienced bugs in them in an automotive application. A bug will block some air causing an inaccurate resistance feed back. The ECM will respond by changing the fuel rate. May not be related, but would only take a sec to take a look...
 
fwiw, I dropped about 300 rpms and about 3 mph from May and replacing the fuel filter seemed to restore both.
 
A simple thing easily missed, the wiring to, or a faulty tacho.

I had a loose earth wire floating around behind the instrument panel, near the tac, and every so often it would touch the input treminal of the tacho and kill the motor for a moment then come good again.

Took days to find it. It was a spare earth wire and the tape had fallen off the lug.

To check yours, just remove the wire from the coil or ignition module and run without the tacho and see if the fault disappears. That's how I found my problem.
 
Chris Dauth said:
A simple thing easily missed, the wiring to, or a faulty tacho.

I had a loose earth wire floating around behind the instrument panel, near the tac, and every so often it would touch the input treminal of the tacho and kill the motor for a moment then come good again.

Took days to find it. It was a spare earth wire and the tape had fallen off the lug.

To check yours, just remove the wire from the coil or ignition module and run without the tacho and see if the fault disappears. That's how I found my problem.
Click to expand...

I had a bad Tach early this year, that is how I found that problem........(I unpluged the Tach wire!!) I never knew a bad Tach could keep you from getting your boat started! But I do now!!
 
I just had a simular problem this past weekend. I was running at 3800 RPM's when all of a sudden my port engine lost about 600 RPM's. I had to push the throttle all the way forward to get it back to 3800rpm's. Then at one point the port engine picked up and I was at 4400 RPM's so I brought the throttle back till I got down to 3800 RMS's. On the trip home I lost RPM's again. This time it never came back and I ran with my port engine throttle all the way forward to maintain a 3800 cruise.
 
sdamato said:
I just had a simular problem this past weekend. I was running at 3800 RPM's when all of a sudden my port engine lost about 600 RPM's. I had to push the throttle all the way forward to get it back to 3800rpm's. Then at one point the port engine picked up and I was at 4400 RPM's so I brought the throttle back till I got down to 3800 RMS's. On the trip home I lost RPM's again. This time it never came back and I ran with my port engine throttle all the way forward to maintain a 3800 cruise.
Click to expand...

When I tryed to increase my RPM'S I got nothing....They now think it is the ping (knock) sensor...or some week fuel...I still have a lot of fuel, so they are going to try to run it off of a 6 gallon can....:smt101
 
The knock sensor is the only thing I haven't replaced yet. Keep the updates coming. I am beginning to hate these Merc Horizon engines...

Thanks,
Steve
 
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