Stubborn Overheating Issue

[twincaman]

Here's one for all you experts. My 1992 175 hp Merc was fine running between 130-140 degrees till one day I got caught in a squall on the lake. Water got very choppy and the boat slammed a couple times pretty hard. On the way back to the dock I noticed the temp going up and it peaked at just over 170 with water squirting out the pee tube normally. Since then the overheating has persisted. I have rebuilt the water pump, checked and replaced the temp sensor, replaced thermostats, replaced poppet valve and even pulled the powerhead and replaced the gaskets on the baseplate. Engine pees nicely and looks normally forceful.. Still runs around 160-170 at medium rpm, but cools down to 150 at idle or high rpm. WTF? Infrared temp sensor indicates the overheating is evenly distributed on both cylinder banks and all cylinders.

[David - WI]

The water is warmer now? Both of the 200's were running 160* when we were out the other day.

[tlwjkw]

160/170 isn't really "overheat" for it actually... but still can't see a reason for a "sudden change" if you did all that??

[bullet123]

Maybe the rough ride messed up the gauge? Try another one

[twincaman]

Maybe the rough ride messed up the gauge? Try another one

I replaced the temp sensor and double checked with an infrared thermometer. High temp is evenly across both cylinder banks. Temp was high months ago when water was much cooler.

[twincaman]

But thanks for all the responses. My buddy the master mechanic is undeterred and we'll continue tearing away at this thing till its resolved. I'll let you know if anything miraculous happens. Next we try a softer spring on the poppet valve and measure the water pressure at the top at speed.