What does a car thermostat do?

Answer 1

The thermostat is a device which blocks the flow of coolant through the radiator until the coolant inside the engine reaches a certain temperature, at which point the thermostat gradually opens to allow enough coolant though to maintain the engine at its optimum operating temperature. The thermostat then controls the flow of coolant to keep the engine at that temperature irrespective of engine load and operating conditions.

Without a thermostat and engine warms up more slowly and will never reach its optimum temperature. This causes increased fuel consumption and increased wear on the moving parts.