 Modern engines run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce the emission of pollutants.While the thermostat is closed, there is no flow of coolant in the radiator loop, and coolant water is instead redirected through the engine, allowing it to warm up rapidly while also avoiding hot spots.The thermostat stays closed until the coolant temperature reaches the nominal thermostat opening temperature.