I have a Mercury 50hp 4 stroke that is overheating after about a min at wot. RPM at wot is right around 5500 for this motor and that is right where she is running. I had just replaced the water pump impeller prior to putting the boat in the water. Does anyone have any ideas as to what might be wrong? I have thought about checking the thermostat but I do not know what a bad one looks like.

I have also checked to make sure the impeller is installed correctly and it is. Also, I do get a good stream from the back of the motor.

Did this happen before you replaced the impellor?

I should also add that this only happens if i run between 3/4 throttle and wot. . . idling and low throttle doesn't cause the warning buzzer to go off.

Did this happen before you replaced the impellor?

It occasionally happened. . . but not like this. I used to get the odd warning bell every once in a while but this is every time I go wot.

Is the engine actually overheating, or is just the warning buzzer going off. The engine should feel considerably hotter if it is getting to the point of the buzzer going off.

Mine started doing that and I just disconnected it cause it was like 400 bucks for a new unit to be installed. As long as the water is pumping out the back I'm a happy guy.

If your t/stat is stuck it would overheat at any speed, if it truly is getting hot you wont be able to get your hand near the engine and it will sound like you are frying bacon.

The best thing for you to do is take it to a good tech and let them look it over.

what year is the motor? I am not sure what years had the problem but there is a rubber grommet under the powerhead that get's pushed out of place and restricts water flow. I have seen this on a bunch of these motors. in order to fix it you need to pull the powerhead and replace the grommet. really a job for a marina to do. symtoms would be occasionally at first then happening all the time. only shows up at high speed. you can try thermo, pump and other items first but this is most likely your problem. definitely do not reccomend disconnecting the alarm buzzer.

I pulled the thermostat and did a boil test and it did not open. Should open at 120 degrees. It never opened even up to 200+ degrees so this will be where I start I guess.

I guess I should add that I had an experienced Merc tech look at the motor first and tell me it was just a bad unit and told me it would be expensive to replace and just unhooked it and told me what to watch. I didnt take the leeroy route and just start cutting wires.