Function

Thermostat
The thermostat regulates the amount of coolant to the engine. The wax body expands when energy reaches it in the form of heat. In modern cooling systems, the thermostat begins to expand when the surrounding coolant temperature is 90°C.

When the wax body expands in the thermostat, a flow of coolant is allowed through the radiator whilst the flow to the by-pass is shut off to speed up cooling. Coolant routed through the bypass returns to the engine without being cooled. Any air is able to leave the system via a jiggle pin. The jiggle pin is in the thermostat.