A car’s thermostat is the component in its cooling system that performs two main functions. It must get the engine to heat up to its optimum temperature as quickly as possible, and then keep the engine at its optimum temperature.
The most popular type of thermostat regulates the flow of coolant to the air-cooled radiator.
Such thermostats use a chamber that contains a wax pellet, which melts and expands at a set temperature.
This process operates a rod that then opens a valve when the operating temperature is exceeded, allowing through more coolant into the radiator. The operating temperature is determined by the composition of the wax.
Listening out for the cooling fan can be a quick and easy way to tell if your car’s cooling system is working at peak efficiency or not.
If you hear the fan being activated on a cool day, or if the fan is in operation after even a short run, there’s a good chance that either the level of coolant in your car has dropped, or there’s a fault with the thermostat.
It’s worth letting the car’s cooling system cool down so you can accurately check the level of fluid in the expansion tank – it should be between and H and L mark on the side of the tank.
If the level of coolant is low, it’s worthwhile checking the state of the car’s rubber cooling pipes.
These lead from the radiator to the expansion tank and from the car’s engine to the radiator.
Vibrations caused when the car is moving can make the pipes wear if they happen to be rubbing against other components, and if there are any bulges in the pipes you should replace them straight away and bleed the system (see section on Bleed screws).
If there are no leaks in your car’s cooling system, the fault will likely lie with the thermostat, which must be changed.
At Haynes, we have guides on how to change the thermostat on most makes and models of car.
Beware though – a replacement thermostat must be the designed for your particular vehicle, because a differently calibrated one could allow your engine to run too hot or cold, and cause irreparable damage.