2000 MAG 454 MPI, loss of power

Hi:
I have an issue with the Stbd motor in my 2000 SeaRay 380DA that I hope someone can help me with.
Both engines long block rebuilds with about 300 hrs since. No issues prior to trip.

Prior to our 800 mile trip from Palm Coast to Gun Cay, Bahamas and back...in both engines: changed the fuel and oil/filters, seawater pump housings and impellers, new alternators and new batteries, new serpentine belts, new IAC's and cleaned the spark arrestors and hauled out for fresh bottom paint. Both heat exchangers serviced with no issues or leaks. Manifolds and risers are less than 1 year old. Total replacement of old "log style" wet exhaust to water lift style with all new hoses and mufflers etc. Props tuned and cutlass bearings are in spec.

The trip was great, zero issues except for one day while in Bimini the Stbd engine stumbled while leaving the harbor at around 1500rpm (sputtered and suddenly revved up to over 2000 rpm without me touching the throttle). It died and I was able to immediately restart it with no further issues all the way back to Palm Coast.

Never use ethanol gas and ran with Lucas fuel treatment the whole trip.

We have been back home for some time and about 2 weeks after the trip, the Stbd engine:
Starts normally, idles normally, runs at 167F max
Wont rev past 3200rpm (no matter how far I push the throttle), seems like it has no power.

I have:
Checked with my Rindal scan tool and the only thing was "MAP sensor voltage low", so I replaced the MAP sensor (even though when testing with vacuum it was within spec)
Checked vacuum in plenum and it has 15" at idle (factory spec)
Changed fuel filter (no water or debris in filter)
Changed spark plugs (they were wet/foul)
Used in-line spark plug tester and plugs were getting spark
Distributor cap has corrosion on each rotor/plug lead point (inside cap) so I am replacing caps and rotors to better OEM brass style (currently has aluminum style).

It seems to me that engine is getting fuel (too much) and it may be weak ignition from the distributor cap corrosion?

Also checked MAP and ECT sensors as per the Mercury Manual and it points to a bad ECM but when compared to the Port engine sensors(which is running great) it also says the ECM is bad so I wonder about these diagnostic instructions in the manual?

Fuel pressure?....injectors? I guess the only way to tell is to remove the intake plenum to get to the fuel rail and check pressure and remove injectors?

Huge thanks to anyone who can help me!!
Thx

 

Hey:
Yes, I ran the non-scan diagnostic steps in the Mercruiser Factory manual and it is getting the 5v reference signal.

I have only owned the boat for less than a year but the previous owner had the injectors service/rebuilt back in 10/2017 (have receipt). Do you think injectors need to be cleaned?
Thx!

 

The "MAP voltage low" code was one time and has not thrown it again. I ran the boat a few days ago with the scan tool in "live" and MAP, TPS, ECT all were giving normal readings as they related to increasing and decreasing RPM's. While idling back to the home dock, the engine almost sounds like it is running on certain cylinders and not firing all?

 

Thx...
Compression was 150psi on all, right before I replaced the plugs (hoping plugs were the issue). The plugs that were in the engine was the NGK non-platinum tip and I replaced with the same ones (are the platinum tip necessary?)

If the new distributor cap and rotor doesnt clear it up, I guess I will pull the intake off and check fuel pressure and pull the injectors
Thx again

 

Thx....the more I read/research, the more it sounds like a fuel pressure regulator issue. Once I have new intake gaskets in hand, I will pull the intake and check the pressure at the fuel rail and then move on to injectors if necessary.

 

Thx....
The vessel build date is 2000 but the engines were factory built in the latter part of 1999 apparently. They both are the 454 CID (7.4L) MAG MPI Horizon (Gen VI) with serial # 0L398572 with the MEFI 3 ECM.
In looking at the factory manual, it looks to me like the schraeder valve is on the fuel rail and buried under the intake? If I dont need to take the intake off at this point, I would rather not.

 

Yeah...reading 370Dancer thread and how he did it
Good Idea!

 

Thx Mitch
According to my Mercruiser Manual for this year and model engine, 15" at idle is the spec.

 

Thx Mitch:
I am in Palm Coast, Fl....just above Daytona. Since I am leaning towards a bad fuel pressure regulator or perhaps the vacuum line to the regulator being bad, it may affect the vacuum in the plenum?