89 Cavalier Overheat

My 1989 cavalier has an overheating problem. It runs fine for about 20 miles then gets really hot really quick. I have replaced waterpump the radiator is in good shap (its not leaking) and no bad hoses and i belive the fan works. Im thinking it might be the thermostat or the sensor by the thermostat. I would like to get some more opinions. Thank You Skyler

Was it overheating before the water pump was replaced? If not, you probably have air in the system. If it was, it may be the tstat, rad could be clogged although it is not leaking, if you have an electric fan, it may not be turning on or the radiator fan switch may be bad. Some vehicles don't have a rad fan sw and rely on the coolant temp sensor which works with the ECM to turn on the rad fan. If the car is getting hot while moving (eg highway speed), I would say the rad is clogged or the tstat is not working properly (after ruling out air in the system and verifying that the fan is kicking on.) If you turn the AC on, it should kick on the fan. This may help determine if the rad fan motor is working. Good luck!

Thanx it was a bad rad