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Cooling Systems: Part 1

The Thermostat:

Just as an athlete needs to warm up when they begin to exercise, your car's engine needs to warm up when it is first started. The thermostat is almost always placed between the engine and the radiator as the coolant flows. The thermostat's main job is to allow the engine to heat up quickly, and then to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. The thermostat controls your engine's warm-up period by recirculating the coolant until it reaches the proper temperature. When the correct operating temperature is reached, the thermostat opens and allows the coolant to go to the radiator.

The thermostat is a temperature sensitive spring valve that remains closed while the engine warms up. The normal operating temperature for most engines is between 180° F and 200° F. When the right temperature is reached, the spring valve opens, allowing coolant to circulate through the radiator to be cooled.

The center of the thermostat has a small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at around 180° F (in a 180° F thermostat which is the most common). A rod connected to the valve presses into this wax and as the wax melts; it expands, pushing the rod out of the cylinder and opening the valve.

The best way to test a thermostat is to place it in a pot of boiling water on the stove. As it heats up to its rated temperature, its valve should open about an inch. If it doesn't, then throw it out and get a new one.

The temperature at which the thermostat is designed to open is called its rating and is usually stamped on the body. The 180° F thermostat begins to open at 180° F and is fully open at 200° F. Different engines use different temperature calibrated thermostats. In addition to maintaining the proper operating temperature, the thermostat also controls the speed of the coolant through the engine. If the coolant moves too fast, it will not pick up as much heat as if it were moving more slowly.

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