1994 Mercury 175 overheating at 1900 rpms

[jstone101]

My 1994 Mercury Black Max 175 has developed an overheat problem at 1900 rpms. I have replaced the water pump, poppet valve, and thermostats. I have also used an infrared gun to confirm the temp on the head. If I run the engine at idle, it runs at 160, WOT also at 160 degrees. The only time the temp will climb is at 1900 rpms. Any thoughts? I have taken it to my mechanic and he is stumped as well. Thoughts are appreciated.

Thank you

 

[CharlieB]

Re: 1994 Mercury 175 overheating at 1900 rpms

When you say you replaced the water pump did you replace both upper and lower housings?

What is the water pumps position, gearcase elevation in the water when running at 1900?

A slight warp in either pump housing and the gearcase rising in the water bringing the pump OUT of the water, begins to suck air into the cooling system. Higher RPM the pump output is still enough to cool the motor.

Install a water pressure guage and watch for an droop in cooling pressure.