100 fourstroke

[R. Blake]

Engine runs hot while idle.Impeller is new.

[boats.net]

Have you checked the pressure control valve and the thermostat, and compression test.

[Ken]

R. Blake,
What indication do you have that the motor is overheating? Temp gauge? Alarm?
Overheating can be caused by a weak water pump, faulty pressure relief (control) valve, stuck thermostat, or clogged up water jackets.
The PRV can get stuck partially open due to mineral deposits or weak/broken spring, and thereby allow cooling water to bypass the water jackets and dump out the relief holes. Remove and inspect/clean per the Yamaha shop manual.
The thermostat can fail to open all the way due to mineral buildup - remove and clean and test in pan of heated water and thermometer per the manual. Fully open should occur at 140 degrees.
Good luck, and let us know what you find out,
Ken K [img]smile.gif[/img]

[R. Blake]

Thanks for the help.I replaced the thermostat,and performed just a general clean up of the whole cooling system.Lots of gunk and impeller pieces.Thermostat was stuck closed. Engine is now running good.