If you suspect the thermostat is no good, replace it with a new one, don't leave it out.
thermostat help fan come on when the car is runnning, keep the engine from over heating
change the thermostat
check your thermostat
If your car is over heating you may want to check your thermostat and replace it may solve your over heating problems
Could be the thermostat not opening
thermostat
2000 Pontiac grand am se keeps over heating I changed the water pump, the thermostat, radiator cap, head, and it is still over heating. make sure thermostat is going the right way , spring gose to the motor
If you are unsure if it is heating because of a water pump or a thermstat, you can find out by removing the thermostat. If it still heats without a thermostat, then you know it is the water pump. Without a thermostat, the water flow has nothing to cause it to heat unless the water pump fails to circulate the water. No circulation of the water would cause the over heating.
check the fan clutch
Thermostat is bad
A thermostat is one cause of over heating engines.The "Related Question" below lists a few other suspects to investigate that can also contribute to overheating.
The function of a car thermostat is to ensure that coolant gets to the radiator when it should. It blocks the coolant from reaching the radiator when the radiator is cold, and then lets it flow once it is heated up.