Re: 1989 135 hp mercury overheating at mid range
How do you know it's overheating? Does the high temp sensor close and the solid alarm sounds then goes off if you slow down or speed up?
Never heard of such a thing.
At low speed, cool/cold temperatures, the thermostat and pressure relief stay closed causing the engine to warm up. At approx 120F the thermostat starts opening regardless of the rpm's. By 130F it should be fully open.
The pressure relief valve is pressure driven. So, if you drive your boat forward with significant speed, regardless of the temperature, sufficient pressure (as a result of your forward speed) will cause water to enter the water pickup tubes in the lower unit of the engine.
This pressure will force the pressure relief valve off it's seat and bypass the thermostat and the volume is like 4 or so times the volume of the water going through the thermostat. You will have immediate cooling and if your stat was your problem your horn will go off in a matter of seconds. BTDT
So, I don't understand the root cause of your problem.
Mark