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What is a Car Thermostat?

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  • Written By: Ron Marr
  • Edited By: Bronwyn Harris
  • Last Modified Date: 21 September 2016
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Those clouds of steam pouring from underneath the hood of the family car could be indicative of several automotive dilemmas. The radiator might have run dry or sprung a leak. The water pump might have failed, or a hose might have ruptured. Just as likely is that the vehicle’s thermostat has gone bad. The car thermostat is a very small device, but it is integral to a vehicle’s heating and cooling system.

A car thermostat — at least on water-cooled vehicles — controls the coolant flow through the engine. When the vehicle is cold, the thermostat prevents coolant from passing from the radiator to the engine. When the vehicle reaches an appropriate operating temperature, the car thermostat opens up and allows the coolant to circulate. This is vital for the survival of the engine. A car thermostat is simple beyond belief, and could be described as part thermometer, part valve, and part switch.

The car thermostat is located at the end of the radiator hose, attached to the engine in a housing secured by several bolts. It is roughly 2 inches (5 cm) in diameter and roughly 2 inches (5 cm) long. When the engine reaches a temperature of approximately 200 degrees Fahrenheit (93 degree Celsius) the thermostat opens. Coolant flows through the cylinder head and is forced through the car’s cooling system by the force of a water pump. The engine cools, the thermostat closes, and the entire process repeats as needed.

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The magic behind the operation of a car thermostat is nothing more than a chunk of wax. The end of the thermostat attached to the engine contains a cylinder; inside the cylinder is a glob of wax that melts at roughly 180 degree Fahrenheit (82 degrees Celsius). When the wax melts it expands, pushing a tiny rod out of the cylinder and against a valve. The valve opens, the coolant is allowed to flow, and the engine runs smoothly and does not overheat. The wax solidifies and contracts when the engine cools, retracting the rod and closing the valve.

Thermostats are one of the least expensive replacement parts on a car, and are available at virtually any auto parts store. This is a good thing, because they need replacing fairly often. Thermostat prices begin under $5 US Dollars (USD) in 2011 prices. They can be much higher, as different cars require different thermostats. Still, even the most expensive thermostat should not run over $20 USD.

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