A car thermostat is installed on a car engine and regulates the engine temperature by opening to allow coolant from the radiator to flow through the engine, cooling it down, then closing again to allow the engine to warm up to operating temperature.
This cycle repeats itself as you drive, keeping your engine at its optimal temperature for driving. Most automobile mechanics will tell you it is not a good idea to drive your car without a thermostat installed.
If your thermostat becomes stuck in the closed position, though, this will cause your engine to overheat and make driving your car impossible. If you are in this sort of a bind and simply have to drive without a thermostat, it can be done.
Driving Without a Thermostat