This illustration shows the basic cooling system function in a vintage Mustang engine. Heat is transferred to coolant in the water jackets. When coolant temperature reaches the thermostat’s rated opening temperature of either 160 or 180 degrees F, the thermostat opens to release hot coolant into the radiator. Coolant from the radiator then flows into the engine’s water jackets. Engine cooling is nothing more than heat transfer to coolant to the radiator to the incoming air stream. The thermostat controls the heat transfer to keep engine temperature constant.