I recently noticed that my coolant temperature started rising to dangerously high temperatures when only driving for about 5 minutes.

I'm not sure if this is connected to the thermostat housing or the temperature sensor, but I'm afraid to drive around now.

During the winter it was fine (probably because engine temps were always cold), but now it is a whole new problem.

Has anyone else had this problem before or know what may fix this problem?

I already flushed the coolant out of the radiator and refilled, but nothing improved. I checked for coolant leaks, but there were none to be found.

I am baffled.

 

I wish I could say I know exactly what is wrong. However, in lieu of that, I point out that the 4g63 engine has an aluminum head on a cast iron block. Aluminum has a different coefficient of expansion meaning the head is moving on the block every time the engine is run. Gasket makers were crowing about being able to make this seal twenty years ago. Now, we don't think much about it. However, overheat a little, get warping of the head and develop problems.

USDM Evos aren't known to have thermostat problems but any thermostat can fail. So, don't mess around, change the thermostat immediately – it is a cheap part. If that doesn't fix it then look elsewhere. Don't warp the head trying to avoid the cost of a thermostat.