Mercrusier 350 MAG MPI - RPM issue under a load

[wbas1]

I am looking for some help on an issue I have been working on for a few months and it has kicked my butt. I have a Mercrusier 350 MAG MPI rebuilt block. The motor at idle has no issue reaching 4k+ RPM's. The issue I am having is when I try to get the boat on a plan, I cannot get the motor above 3k and it will not plan. All of the trouble shooting I have been done was with a 5 Gallon tank of gas.
-Fuel pressure at the rail is 38psi. When I disconnect the vacuum at the rail, pressure goes to 43psi. When I disconnect the vacuum from the High pressure pump the fuel pressure reaches close to 50psi.
- Acceleration at the dock is perfect. I understand that is because there is no load and does not require much fuel.
- Bravo III outdrive 2 to 1 ratio
- Fuel cool setup
-Duo Pro 20 Pitch - In good condition

Items that have been replaced

1. Map Sensor
2. IAC
3. TPS
4. Distributor/Cap (Flat Cap)/Plugs & Wires
5. Coil & ICM
6. Temp Sensor
7. High & Low Fuel pumps
8. Water Separator - No water present
9. Cleaned Throttle body
10. ECM for 5.7 closed cooling
11. Fuel Pressure Regulator at the rail

Any help is greatly appreciated

For some reason the original motor had an ECM for a 6.2 Bravo where I was able to get the boat on a plan only when I disconnected the MAP sensor. I was told by mercury that was because it put the ECM in a loop and was using a default program. They recommended that I get the correct ECM for my motor. That is why I have a new ECM installed.

 

[QBhoy]

What boat is she trying to push ? Was the cap and rotor replaced recently? What plugs do you have in her ?

 

[itsathepete]

Ok tell us a little bit more of the backstory. You state you have a rebuilt engine in the boat now. Completely rebuilt or short block? Did you rebuild it or buy it that way. Was it installed when you got it? Did you install it? Did you have it installed? Is it the same engine that had the 6.2 ECM that would plane out with the MAP sensor disconnected or was that a different engine? If it is a different engine, why was it replaced? Please explain

 

[alldodge]

Ah, lets start with more centered info on what were working on

Motor serial number?
What boat is it in?

Being a 6.2 it can have a 555 ECM or PCM

 

[wbas1]

What boat is she trying to push ? Was the cap and rotor replaced recently? What plugs do you have in her ?

27.7 Baja Crusier. everything was replaced. not sure at this moment

 

[wbas1]

Ok tell us a little bit more of the backstory. You state you have a rebuilt engine in the boat now. Completely rebuilt or short block? Did you rebuild it or buy it that way. Was it installed when you got it? Did you install it? Did you have it installed? Is it the same engine that had the 6.2 ECM that would plane out with the MAP sensor disconnected or was that a different engine? If it is a different engine, why was it replaced? Please explain

I purchased a Long block. It came with heads, timing chain & water pump. I installed the motor this winter. Same engine that had the 6.2 ECM on it

 

[wbas1]

Ah, lets start with more centered info on what were working on

Motor serial number?
What boat is it in?

Being a 6.2 it can have a 555 ECM or PCM

original serial # 0W043043. Baja Crusier 27.7

 

[wbas1]

What boat is she trying to push ? Was the cap and rotor replaced recently? What plugs do you have in her ?

27.7 Baja Crusier. Yes they were replced. I have to look at the plugs since they were replaced when I had it at a mechanic

 

[wbas1]

Ah, lets start with more centered info on what were working on

Motor serial number?
What boat is it in?

Being a 6.2 it can have a 555 ECM or PCM

This was the ECM I had on it at 1st when Mercury told me it was wrong.

 

[alldodge]

Think I know part of the problem
Baja dry weight is 6000 LB, 9.5 Beam

The ECM 865845 in pic is for a 6.2 MPI, Bravo open cooling S/N 0W029411 and above, which is good for your old motor
Long block motor installed is a 350 Mag MPI, Bravo closed cooling using same S/N would use ECM 865839

The 6.2 MPI is 320 HP
The 350 MPI is 300 HP

The 6.2 has more low end torque (not a lot more) then the 350 and that's what is needed to get the boat up on plane

So missing 20 HP, and the rest may be to timing, but could also be other things.
To help narrow it down, have you owned the boat long?
How did it perform before?

 

[wbas1]

Think I know part of the problem
Baja dry weight is 6000 LB, 9.5 Beam

The ECM 865845 in pic is for a 6.2 MPI, Bravo open cooling S/N 0W029411 and above, which is good for your old motor
Long block motor installed is a 350 Mag MPI, Bravo closed cooling using same S/N would use ECM 865839

The 6.2 MPI is 320 HP
The 350 MPI is 300 HP

The 6.2 has more low end torque (not a lot more) then the 350 and that's what is needed to get the boat up on plane

So missing 20 HP, and the rest may be to timing, but could also be other things.
To help narrow it down, have you owned the boat long?
How did it perform before?

I bought the boat last September with the frozen motor. I was never able to test the boat. The old motor says 5.7 on the block & that is what this one is. I am using

Mercury - Electronic Control Module - Fits 2004 MCM 350 Mag MPI Bravo with Closed Cooling - 865841T02 SKU: 865841T02

 

[wbas1]

I bought the boat last September with the frozen motor. I was never able to test the boat. The old motor says 5.7 on the block & that is what this one is. I am using

Mercury - Electronic Control Module - Fits 2004 MCM 350 Mag MPI Bravo with Closed Cooling - 865841T02 SKU: 865841T02

This is what I bought:
Remanufactured 5.7L Vortec Marine Base Engine. Replaces years 1997-2015. Base engine package includes: longblock, oil pan, timing cover, valve covers, and harmonic balancer.

 

[wbas1]

I bought the boat last September with the frozen motor. I was never able to test the boat. The old motor says 5.7 on the block & that is what this one is. I am using

Mercury - Electronic Control Module - Fits 2004 MCM 350 Mag MPI Bravo with Closed Cooling - 865841T02 SKU: 865841T02

Not sure if I understand because when I had the old ECM on the boat & I disconnected the MAP sensor, I was able to get the boat on a plan.

 

[Rick Stephens]

Might run a compression check to make sure of baseline components.

Double check firing order routing of spark wires.

 

[alldodge]

ok, 350 to 350 swap,old motor did have the wrong ECM, got it
No HP change

This being a new to you boat then we need to determine, drive ratio, prop pitch and if the hull is water logged

Not sure if I understand because when I had the old ECM on the boat & I disconnected the MAP sensor, I was able to get the boat on a plan.

Disconnecting MAP makes the motor run outside of the ECM parameters and gets extra fuel and other things.

Have you tried moving the Dizzy ever so slightly in either direction?
Move it slightly, run the boat, if no change go back to starting place then slightly the other direction. If there was a gain, then ever so slightly a tad more

 

[itsathepete]

The 6.2 uses the same block as the 5.7 so the original motor could have possibly been a 6.2 and you didn't know that. It would make sense if it had a 6.2 e c m. It could also explain why you can't get the 5.7 to run properly using all the components for the 6.2

 

[wbas1]

The 6.2 uses the same block as the 5.7 so the original motor could have possibly been a 6.2 and you didn't know that. It would make sense if it had a 6.2 e c m. It could also explain why you can't get the 5.7 to run properly using all the components for the 6.2

I just confirmed with Mercury and the original engine was a 5.7L 300HP. That is what I have now

 

[wbas1]

ok, 350 to 350 swap,old motor did have the wrong ECM, got it
No HP change

This being a new to you boat then we need to determine, drive ratio, prop pitch and if the hull is water logged


Disconnecting MAP makes the motor run outside of the ECM parameters and gets extra fuel and other things.

Have you tried moving the Dizzy ever so slightly in either direction?
Move it slightly, run the boat, if no change go back to starting place then slightly the other direction. If there was a gain, then ever so slightly a tad more

I have tried that before but I thought you cannot adjust that at all. I can try it again. The drive is 2 to 1 with a 20 pitch

 

[wbas1]

Might run a compression check to make sure of baseline components.

Double check firing order routing of spark wires.

I have 150PSI in all cylinders and I have triple checked the firing order according to the Dizzy Cap numbering

 

[alldodge]

I just confirmed with Mercury and the original engine was a 5.7L 300HP. That is what I have now

The motor may have been rebuilt once before and a 383 (6.2) rotating assembly may have been installed prior. Not saying it was, just a possibility

I have tried that before but I thought you cannot adjust that at all.

Normally it is not done, but can be moved ever so little, a degree or 2 can change things. There is a Merc Bulletin which shows how to build a $10 tool to get the timing exact. I can post if you want it