Overheating '99 Merc 50 HP 4 Stroke

I recently bought a nice little Jones Brothers Bateau 176 with a 1999 Mercury 50 hp 4 stroke motor. When I run the motor at idle for just a few minutes it overheats and the alarm comes on. The boat runs fine at cruising speed. I recently replaced the impeller (which was in dire need of replacement) but the alarm still sounds after a few minutes of operation at idle.

I'd appreciate any ideas as to what the problem may be and how to fix it.

Thanks

 

if still overheating after replacing the impeller move to the thermostat and with the t-stat out check for good water flow

 

If the impeller was missing any pieces you need to fish them out may have a partial clog somewhere in the system. Are you getting a good stream of water at idle? Have you checked the intake? A partial clog there may make it too hard for the impeller to move the water at low speeds.

 

Thanks

Thanks for the responses about the thermostat. I have been heading in that direction and just got to the dealership today to get a new thermo. As soon as I get home (I'm at the beach with 30 kids right now), I'll replace it. I will try testing the engine with no thermo installed and then with the new one. I bought the engine like a pig in a poke and have no idea about previous maintenance issues. I suspect just from the dirty oil in the lower unit that I have some "opportunities" to correct past mistakes.

Now, does anyone have any ideas about repairing rotten plywood on my foredeck ...

 

Good News!!! I replaced the thermostat this afternoon and the engine runs like a top! I'll be taking it out on B. Everett Jordan lake Sunday for a spin (and to try my hand at striper fishing). Thanks for your suggestions.