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MerCruiser 7.4 MPI (1998) - Won’t Rev Past 3000 RPM - Looking for Ideas

S

SaschaH

New member
I’m hoping someone has run into this before.

I have a 1998 MerCruiser 7.4 MPI that runs and idles fine, but it will not rev past about 3000 RPM under load. It feels like it hits a wall and just won’t make any more power.

Here’s what I’ve checked so far:
  • Initially had fault codes for:
    • TPS (Throttle Position Sensor)
    • Idle Air Control (IAC) valve
    • Coolant Temperature Sensor
    • Knock Sensor
  • Replaced all of the above.
  • No fault codes anymore.
  • Compression is good on all cylinders.
  • Fuel pressure is within spec.
The engine starts easily and runs smoothly, but once it reaches around 3000 RPM, it simply won’t accelerate any further. It doesn’t really backfire or run rough—it just seems like something is limiting the engine.

What else would you check?

Has anyone experienced something similar on a 7.4 MPI? Any suggestions for diagnostics or common failures would be greatly appreciated.


Thanks!
 
Is this a new to you boat?
You say fuel pressure is within spec, is it 43 psi?

Put a fuel pressure gauge on the rail and see what the pressure is from idle to WOT
 
AllDodge said:
Is this a new to you boat?
You say fuel pressure is within spec, is it 43 psi?

Put a fuel pressure gauge on the rail and see what the pressure is from idle to WOT
Click to expand...
Thank you for the response.

Yes, it’s a new-to-me boat.

I checked the fuel pressure at the rail. At idle it’s around 40 PSI. In neutral, the engine revs up to about 5000 RPM, and the fuel pressure stays fairly steady at around 37 PSI.

The problem only occurs under load. Once the boat is in gear, the engine won’t go past about 3000 RPM. I can do the same test in gear, it was just really jobbi the last days. Fuel pressure doesn’t appear to drop off significantly, so it doesn’t seem like it’s starving for fuel.
 
For me it looks like a fuel pressure issue if your gauge is accurate. It takes very little gas to rev the motor when not under load, and a lot of gas under load

Should see around 40 psi with key on motor off, fuel can drop off a little when pump stops but should not drop much.

Motor running 38 to 43, remove vacuum line off regulator it should go to 43. Under load it should be 43 at 3000 rpm and above
 
AllDodge said:
For me it looks like a fuel pressure issue if your gauge is accurate. It takes very little gas to rev the motor when not under load, and a lot of gas under load

Should see around 40 psi with key on motor off, fuel can drop off a little when pump stops but should not drop much.

Motor running 38 to 43, remove vacuum line off regulator it should go to 43. Under load it should be 43 at 3000 rpm and above
Click to expand...
Thank you again for your repsponse. IT was a calm weekend so i did some tests under load.

Same pressure like in idle, Just at 29.000 30.000 RPM it stops.

1786385042615.png
1786385049280.png
 
Just at 29.000 30.000 RPM it stops
Click to expand...
I cannot read the pic, print is to small.

With it that low need to test fuel pump and regulator. Can place a valve on the line going back to the fuel filter and with pressure gauge on the rail start closing off the valve slowly. Turn the key off/on to have the pump cycle. If you get up to 50 psi stop, the pump is ok the Reg is the issue

I would verify with a pressure gauge on the rail
 
Could be a varnish blockage in fuel system, evaporated gas can plug the lines blocking flow even though the pump has correct pressure. When you move the throttle past where 3,000 normally has it under a load does it start bogging from butterfly being full open for only the 3000rpms.
 
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