We offer thermostats from AC Delco, Motorcraft and Stant. The thermostat's main job is to control the flow of coolant through the engine. The thermostat closes to allow the engine to heat up quickly. Next the thermostat will open to allow coolant to flow for cooling the engine. The thermostat acts as a temperature manager working to to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked -- all of the coolant is recirculated back through the engine. Most thermostats aim for 180 or 195 F.