40hp Mercury OB alarms and quits WOT

[lakeboat9]

We have a late 90's (I think) 40hp Mercury outboard on a 28' Fisher pontoon boot. Runs good all day at lower rpm. If you run it at too high an rpm for too long an alarm will sound and it will only idle. If you shut it off for a while, you can crank it up and take off again, and if you stay at a lower rpm it will run all day. Yesterday I ran it at WOT trying to beat the rain and it was fine, but I was within sight of the dock when I turned around. I assumed it was overheating but it is shooting water out. I go back to the lake in two weeks and I'm not sure where to start looking.

 

[vance14]

Re: 40hp Mercury OB alarms and quits WOT

If it just started doing this it sure sounds like your thermostat is sticking closed.If it is a 3 cyl it is easy to change, I dont know anything if it is a 4 cyl. If it has always done this then the motor is over propped and not reaching the required wot rpm.


vance

 

[lakeboat9]

Re: 40hp Mercury OB alarms and quits WOT

I will check the thermostat. The prop is the original and the boat ran perfectly for years. This problem began in the last couple of years.

 

[joed]

Re: 40hp Mercury OB alarms and quits WOT

When was the impeller last changed? That would be my first thought.

 

[lakeboat9]

Re: 40hp Mercury OB alarms and quits WOT

I'm not sure. My plan at the moment is to put a water pump kit and a thermostat in it next weekend. If that doesn't fix it then I will try to see if the water inlet comes out of the water at WOT and if the motor can be lowered.