Overheating '99 Merc 50 HP 4 Stroke

[taintso]

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

[redtech]

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

[kenJ]

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.

[taintso]

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 ...

[taintso]

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.