Located between the engine and the radiator, this little temperature-sensitive spring valve stays closed during engine warm-up, preventing coolant from leaving the engine and circulating through the radiator until the correct running temperature is achieved. Once the temperature of the coolant rises to between 180 and 195 F (82-91 C), the car thermostat starts to open, allowing fluid to go through the radiator to be cooled. And by the time the coolant reaches 200 to 218 F (93-103 C), the auto thermostat is open all the way.
The trick of the auto thermostat lies in the tiny cylinder at the engine-side of the device. The cylinder is filled with a wax that usually starts to melt at 180 F. A rod connected to the valve presses into this wax. And as the wax melts, it expands, pushing the rod out of the straw. This process is also used in automatic openers for greenhouse vents and skylights.
Various engines use different auto thermostats. There are some high-ranging thermostats that maintain engine operating temperatures above 2,000 F. This causes the engine to burn up more pollutants and helps in emissions control. The range for a specific auto thermostat depends on the type of the engine, load requirements, weather, and other factors. Most of the auto thermostats are "pellet type" - the name comes from the wax pellet that expands as the engine coolant warms. This expansion forces the valve to open. Auto thermostats usually get "stuck shut" as it cuts off its cooling capacity of the radiator. It often occurs after an engine has overheated because of water pump failure or a developing coolant leakage. So it is important to have your thermostat checked to avoid engine overheat, engine wear and excessive waste fuel.