An engine thermostat is a component that can regulate coolant flow through liquid cooled engines. There are several different thermostat designs, though they are typically contained within the cooling system and submerged in coolant. At low temperatures they will tend to remain closed, blocking the circulation of coolant through the system. This can allow the engine to warm up and operate normally, at which point the thermostat may open to allow free coolant circulation. Poor fuel economy and other issues may arise if an engine thermostat is stuck either open or closed.
A typical engine thermostat is composed of a thermostatically operated, spring loaded valve. The valve is usually actuated by a device composed of a pellet of wax within a sealed cylinder. When the wax warms up it can melt, expand, and force the plunger up to open the valve. The wax may also contract when it cools, pulling the plunger down and closing the valve again. This can regulate the engine temperature to within a particular range, allowing for optimum operation.
One purpose of an engine thermostat is to help the engine warm up quickly. Most liquid cooled automotive engines use a heat sink, such as a radiator, to lower coolant temperature. When hot coolant passes through thin radiator tubes, heat exchange may occur and lower its temperature. Without a thermostat, engine coolant could circulate through the radiator as soon as a vehicle is started, preventing it from ever warming up. This could keep the engine from reaching operating temperature, or just prolong the warm up period.
If an engine thermostat is stuck open, a similar problem may occur. The coolant will tend to take very long to warm up, and engine performance may suffer. This could be noticed in reduced gas mileage or other issues. Since hot engine coolant can also pass through a heater core to warm the passenger compartment, a stuck thermostat may result in poor performance of this system as well.
More severe consequences may arise from an engine thermostat that is stuck closed. If the thermostat never opens, coolant can be prevented from circulating through the radiator. This can then cause the engine to overheat, resulting in irreparable damage to a variety of systems and components. The head gasket may fail, or other components could be damaged and require costly repairs.
Replacing an engine thermostat is often an easy operation, though some vehicles can present a challenge. Many thermostat housings are located on the engine block and connected to the upper radiator hose, making them easy to locate and replace. Other vehicles have thermostats that are in more challenging locations, such as on the underside of the engine. In either case, replacing an engine thermostat is typically a matter of removing the housing, installing a new unit, and then bleeding any air out of the cooling system.