I have a 2002 Chevy Impala with the 3.4L engine. The car has 180K+ miles on the original engine. The coolant temperature started fluctuating recently. Going from 200 degrees up to 240 sometimes 260 degrees.
I recently had the check engine light on with the P0420 code which indicates a bad catalytic converter, I had that replaced and I was hoping that maybe the lack of exhaust flow through the converter was causing the overheating. It is still fluctuating however after the converter replacement.
The coolant temperature seems to go up when the car is idling but when moving it starts to go back down to normal. I had the cooling fan assembly replaced last year after one of the fans failed. These are less than a year old and are both operating as expected.
A little history on the engine. It originally had that crappy DexCool in it that then ate away at the cheap plastic head gasket causing oil to leak into the coolant system. This caused the coolant to turn into ‘mud’ within the radiator and eventually the head gasket failed completely and I had to have that replaced along with putting in ethylene glycol to replace the DexCool. Now when the coolant system was flushed, I am not completely sure that all that ‘mud’ was flushed out of the radiator. So the radiator may not be operating at full capacity. In fact, driving through the mountains this summer going up inclines, the car would tend to start overheating.
So thoughts on what might be the issue, radiator, thermostat, sensor, water pump, something else? I’m pretty sure it’s not the sensor because sensors don’t fail in this manner (they fail either high or low). I’m leaning toward the radiator because once I get some air flow across it, the coolant temp gets back to normal.
Also, the A/C is sometimes working great and blowing really cold and other times it is just slightly cold.