An expansion tank is one part of a complex overall car cooling system that’s designed to keep the engine operating at exactly the right temperature.
In the sealed cooling system that’s present on most cars, it is partially filled with air.
The air in the expansion tank provides a ‘cushioning’ effect that allows the heated coolant to expand without causing the cooling system to fail.
This is to prevent the system from becoming over-pressurised as the coolant heats up and expands.
The expansion tank affords home mechanics an easy visual way to keep an eye on the cooling system of their vehicle, because if the level in the expansion tank drops, there may be a leak somewhere, either in the pipework or, worse, in an engine gasket.
It also allows you to keep an eye on the state of the coolant contained in your vehicle’s cooling system.
It’s also a good idea to keep a regular eye on the state of the car’s rubber cooling pipes each month.
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.