115 overheating

[BTruaxjr]

My Mercury 115 started overheating today, I was on the river which was full of debris but the pee hole was still working. When I got home I took air hose and blew everthing out, air blew thru OK. Would pee hole still work if t-stat was bad?

 

[Texasmark]

The tell tale hose comes from the exhaust cooling manifold on the left side of the engine. It's not part of the block cooling system where the thermostat and pop off valve control flow through the block. At low RPMs the thermostat controls block temperature to a max of about 145*F. Above 2500 RPM the water pressure is strong enough to force the poppet off its seat and the increased flow is not thermostat controlled and usually is roughly 10*F above the temperature of the water where the engine is running. Since the ambient water temp and slight increase going through the engine is less than the opening temp of the stat, it remains closed at the higher speeds.

The 115 has a temp sensor embedded in the water jacket cover on the rear of the engine....tan wire. If it closes you get an alarm, solid blast.....I think I recall the closing temp is around 195F. If it occurs at speeds below 2500 expect the stat to be the problem. If it occurs above 2500 and goes away when you reduce RPMs below 2500 then expect the poppet to not be unseating as it was designed to do.

If you got your cooling loop clogged with debris then what I would do would be to remove the water jacket cover (with the spark plug holes) and do a good flush, both via your flush adapter (you put over your water cooling pickups on the LU when flushing/running the engine out of the water), and directly into the exposed engine block interior, with the garden hose, then put the cover back on with a new gasket.

If debris is your problem, then don't stop the flush effort till you see debris on the driveway.