Car Overheating with thermostat but running fine without a thermostat
 

Turns out it was the thermostat. I drove it without the thermostat for about half a day and the car never overheating or spilled coolant from the radiator cap like it did before. Once I put a new thermostat in it went back to overheating and spilling all my coolant from the radiator cap. I ended up taking the thermostat out again and the car runs fine again. I know that I shouldn't drive it without the thermostat but I don't know how to fix this problem. Any suggestions?

It's not opening up.

Possible top of radiator is filled with corrossion.

I had the same problem. Replaced the radiator. Problem gone.

Had a car a while ago that the head gasket was leaking compression into coolant and the air would gather at thermostat and thermostat doesn't get hot enough to open with air around it.

You maybe putting the thermostat in backwards also.