My 2000 GMC Sonoma 4.3L only overheats when I am sitting with the engine running. It runs at normal temperature while driving. I’ve change the coolant temperature sensor, flushed the coolant system and replaced the intake gaskets because it did have a small leak. I thought replacing the gaskets would definitely fix the problem but it still starts overheating when sitting still for 10 to 15 minutes. I have also replaced the coolant clutch fan and thermostat.am also getting really crappy mileage,like 10-10 mpg.):
While reading this my first thought was the cooling system clutch fan. I continued reading to see you too had thought of that and already replaced it. My next thought would be to confirm the engine temperature with a hand held temperature gun like this one you can pick up pretty reasonable if you don’t already own one.
Once confirmed you may need to test/replace the radiator cap. The cooling system won’t work properly if the radiator cap opens early just like the thermostat. Coincidentally if the thermostat is stuck open or completely removed the results will be the same as what you are experiencing. But since this was happening before you touched the thermostat I would lean more toward the radiator cap. Cheaper to just replace the radiator cap if you don’t already have a Radiator Cap Tester.
AC Delco RC95 GM Original Equipment Radiator Cap – Fits 2000 GMC Sonoma 4.3L
There is actually a TSB on this as far as overheating being caused by the radiator cap. Take a look:
