 A correctly designed thermostat will never be fully open or fully closed while the engine is operating normally, or overheating or overcooling would occur.Engines which require a tighter control of temperature, as they are sensitive to "Thermal shock" caused by surges of coolant, may use a "constant inlet temperature" system.In this arrangement the inlet cooling to the engine is controlled by double-valve thermostat which mixes a re-circulating sensing flow with the radiator cooling flow.