HOW A THERMOSTAT WORKS

Open and closed case

When the coolant is cool, the immersed thermostat remains in a closed position. This allows the water pump to circulate rapidly, warming coolant around the engine, but not through the radiator. Once operating temperature is reached, the thermostat opens, allowing coolant to flow through the radiator. Note that the piston remains stationary.

Circuit diagram

In a basic cooling system, the thermostat initially blocks coolant passage through the radiator, while keeping it going around the engine. Once at opening temperature, coolant can then flow freely throughout the entire circuit, under the influence of the water pump, thus through the radiator, to be cooled by the fan.

Opening test

Checking the opening temperature of a thermostat is a job you can get away with performing in the family kitchen. Dangle the thermostat on a piece of thread, or similar, and place a thermometer in a pan of cold water on the stove top. Turn on the hob and raise the temperature of the water until the thermostat opens, at which point, check the temperature reading. Simple.