Mercury MAG 350 MPI Horizon loss of HP

[TOM757]

Running into an issue with my 2008 mercury Mag 350 horizon inboard (OW697731) (3-3500349 S) with Borg Warner 71C. It seems like I have loss of HP hoping to get some positive direction from the forum. Thanks in advance.

Symptoms:
- Starts up great, idles perfect, no issues going on plane. once on plane after a few minutes cruising at 20-23kts the speed decreases sometimes losing a full 10kts. no change in throttle.
- get an alarm while increasing Rpm above 2500RPM while in neutral only trailer running on hose).
- intermittent alarm during start, not every time.
- when low load (1 person & low fuel) at WOT the boat experiences a "sneeze"/ hiccup, when RPM lowered and no issues.

What I have checked:
-Fuel pressure >35psi is in spec at idle and high RPM in nuetral
-fuel pressure regulator checked
-new distributer cap
-new TPS
-new MAP
-all fuel filters changed
-alternator producing lower than expected potential (12.3V @ idle and 12.8 @ 1500RPM) will change this week
-flame arrestor cleaned
-plugs show no sign of fuel starvation or overheating
- anti syphon vents cleaned.
- new fuel

No code reader here for the ECM 555 so trying to troubleshoot without it for now, unless someone has one handy in Southern California.

Open to all inputs here and thanks again!

 

[Fun Times]

No code reader here for the ECM 555 so trying to troubleshoot without it for now, unless someone has one handy in Southern California.

Just wondering what city?
Here's an option to rent one for a week if you have a laptop, https://www.ebay.com/itm/284931466798?hash=item42573d262e:g:2VEAAOSwJNdi-B6m&amdata=enc:AQAHAAAAoBybZjE8lf86SYL6BhLZpBsdQ77PayFAE6YTvUaddDrnuWUyTf0FUJf7MlhK893YrtL6aTiLlBQiIfntsGZYKtYpQ2IPD8ycihQvdYjkfhuo7dDuAPjvr/qkKQowkLQHXWqfjzC4QdKeKwDQgf0/HM3REhVO9Zt+4+UJWoy20Ot9bjqMc1PFDlV+xkNSrqHyE8I6HvbJ+lK5yzjb0j6lad8=|tkp:Bk9SR4CW6NXUYA

@alldodge , you honestly should get in on this idea ^.

 

[alldodge]

- get an alarm while increasing Rpm above 2500RPM while in neutral only trailer running on hose).

That is the "In Gear" switch most likely. There is a second switch on the shift plate which sounds an alarm if the motor is not in gear. Not an issue if switch is there. Also running on a hose above 1500 can burn your impeller

-Fuel pressure >35psi is in spec at idle and high RPM in nuetral
-fuel pressure regulator checked

Pressure should be 43 psi at 1800
What did you do to check pressure regulator?

 

[QBhoy]

Hi. I know that similar aged 4.3 mpi have a soft limiter in neutral. Won’t rev beyond about 2500. Earlier don’t. But certainly that 2007 mpi did.
Not sure about the other issues, but you say about the diz cap being replaced…was this just recently and part of the trouble shooting ? If it’s being ruled out as ok, because it was replaced last year or something though…I wouldn’t. Best of luck.

 

[TOM757]

@Fun Times I will look into renting the Rinda! thanks.

@alldodge Maybe my pressure is low, my "clymer" spec was 35psi but sounds like It should be higher.

For checking the regulator I pulled vacuum line off and got an extra 10psi.

@QBhoy For the distruster cap, the error was present before the replaced cap.

Thanks all!

 

[TOM757]

If the culprit is fuel pressure, I can isolate downstream of the fuel pump with a external jerry can, would the high pressure pump that would suggest those symptoms and would it throw an code?

 

[alldodge]

@alldodge Maybe my pressure is low, my "clymer" spec was 35psi but sounds like It should be higher.

The clymer book is good to use as a knee pad but no good for your motor, need a factory manual number 31. Fuel pressure should be 43 psi

For checking the regulator I pulled vacuum line off and got an extra 10psi.

That isn't how the Reg is checked. Remove it and use a compressor with air regulator. Use a blow gun is good with a rubber tip. Remove the screen on the underside of the Reg and press the tip to the small hole. Increase the air pressure from about 20 psi and Reg will open around 43. If it opens before 40 or so its bad. My guess is it is bad