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Beginner's Guide: What Is an Engine Thermostat (And What Does It Do)?

What Is an Engine Thermostat (And What Does It Do)?

A thermostat is a device that reacts to temperature to open a valve, or turn on a switch. A car’s engine thermostat is the component in its cooling system that opens and allows coolant to circulate once the motor is warmed up. This gives two main benefits: 1) It allows the engine to heat up to its optimum temperature as quickly as possible, and 2) it keep the engine at its optimum temperature while running.

Most thermostats regulates the flow of coolant to the engine's radiator, where is is cooled and returned to the motor. These thermostats use a chamber that contains a wax pellet, which melts at between 175 and 200 degrees, releasing a spring. This operates a rod that then opens a valve when the operating temperature is exceeded, allowing the hot coolant into the radiator. The operating temperature is determined by the composition of the wax. 

If your temperature gauge never rises above the 1/4 mark, or your car's heater only blows cold air, your thermostat may be stuck opened. If your car seems to run hot even on cool days, or overheats, it may be stuck closed. To may things more complicated, it is possible for a thermostat to fail, and only stick opened or closed sporadically, which is much harder to diagnose.

Your first step ought to be making sure the cooling system is full of antifreeze. Next, if you have an electric fan, make sure it is working properly, and the accessory drive belt is turning the water pump. If there are no leaks in your car’s cooling system, and the fan and pump are working, the fault will likely lie with the thermostat, which must be changed. 

At Haynes, we have guides on how to change the thermostat on most makes and models of car, and it is typically not a difficult job for a home mechanic.

Beware though – the replacement thermostat must be the designed for your particular make and model of vehicle, because a differently calibrated one may allow your engine to run too hot or cold.