THERMOSTAT
The thermostat is a temperature-sensitive valve in the cooling system that opens
and closes according to internal engine temperature to regulate the flow of
engine coolant through the cooling system, radiator, and back to the engine. This
allows the engine to be controlled and run at factory-specified temperature
parameters, ensuring that the engine runs at optimum performance and
efficiency.
As illustrated below, the thermostat is a valve. The opening and closing of the
valve is controlled by a bi-metallic spring that is sensitive to temperature
changes. When the cooling system is cold the spring contracts, shutting off
coolant flow and accelerating engine warmup. Once the coolant temperature
reaches a predetermined operational temperature (usually 212° F) the spring
contracts (valve opens), and coolant flows freely through the system. When the
valve shaft of the thermostat wears out (or the spring breaks, bind, or wears out),
this causes the thermostat to stick closed, then overheating occurs.