Monsoon 340 loosing power at high RPMs

[FlightTime]

Crew, I had trouble with the 340 Monsoon in my 04 23LSV for the first time today. After running at higher rpms for about 10mins it started running roughly and loosing power, I brought the throttle back and the boat ran fine at lower speed and RPMS...happened twice, both times about 10min into the higher speed stretches. I replaced my plugs, cap, and wires before the season began and had no issues for the first 5 hours. I replaced my fuel filter and cleaned the air filter out as soon as I got home today and also added 2 cans of SeaFoam to the fuel. The only other new variable here is that I put about 30gallons of Rec90 in the boat 2 weeks ago before I put it away. I had always ran regular 87 previously...bad gas? Any help appreciated!

[formulaben]

What was your water temp indicating?  When was the fuel filter changed last?  When was the impeller changed last?  No beeping during the issue?

There's probably a hundred reasons it could do this, but if my memory serves correctly I read about this type of scenario often, and usually it is one of 3 things:

1. Fuel issue; a bad fuel pickup, bad fuel filter, blocked fuel pump/inlet or dying fuel pump. Fuel flow issues makes it run rough at high power settings but many times it will run just fine at high RPM in neutral; a good check is conducted under load.
2. A bad water temp sensor.  ECM induces limp mode due to a sensed overheat condition; limp mode limits engine RPM to a pre-set limit, I think 2000-3000 rpm? 
3. Overheat condition. ECM induces limp mode due to high actual water temp.  There is a problem with water flow (bad impeller, circulation pump, or system blockage.)

Either way, both items are cheap so if the fuel filter didn't fix it, then I would definitely replace the water temp sensor (#3 below) before you do anything else.  FYI, the temp sensor is separate from the one that gives you info on the dash...if you see that you're running high water temps then you have a cooling problem; if it's going into limp mode and temp looks good, then it could be a bad ECM water temp sensor.

[FlightTime]

F

17 minutes ago, formulaben said:

What was your water temp indicating?  When was the fuel filter changed last?  When was the impeller changed last?  No beeping during the issue?

There's probably a hundred reasons it could do this, but if my memory serves correctly I read about this type of scenario often, and usually it is one of 3 things:

1. Fuel issue; a bad fuel pickup, bad fuel filter, blocked fuel pump/inlet or dying fuel pump. Fuel flow issues makes it run rough at high power settings but many times it will run just fine at high RPM in neutral; a good check is conducted under load.
2. A bad water temp sensor.  ECM induces limp mode due to a sensed overheat condition; limp mode limits engine RPM to a pre-set limit, I think 2000-3000 rpm? 
3. Overheat condition. ECM induces limp mode due to high actual water temp.  There is a problem with water flow (bad impeller, circulation pump, or system blockage.)

Either way, both items are cheap so if the fuel filter didn't fix it, then I would definitely replace the water temp sensor (#3 below) before you do anything else.  FYI, the temp sensor is separate from the one that gives you info on the dash...if you see that you're running high water temps then you have a cooling problem; if it's going into limp mode and temp looks good, then it could be a bad ECM water temp sensor.

Engine temp, and oil pressure were all in the normal range. I replaced the impeller about 40hrs ago, no idea when the fuel filter was changed. There was no beeping or any message displayed...thanks for that insight, hoping it’s fuel filter related but was going to look into the fuel pump/injectors and pickup next. Thanks for the info on the temp sensor!

[MadMan]

I would suggest connecting a fuel pressure gauge to the schrader valve (in the photo, lower right) to monitor what is going on when the problem occurs. You'd have to remove the engine hatch to see it, or use a long hose.  This would check several fuel related problems at one time: pump, pickup, filter, regulator.

High RPM/high load are where both the fuel and ignition systems are taxed the most.

Edited May 14, 2018 by MadMan