The purpose of the expansion tank is to protect and store excess coolant. It is a reservoir that stores the overflow of coolant caused by expansion when the liquid has reached a high temperature; without an expansion tank this excess coolant would leak onto the road. Coolant (an equal mix of antifreeze and water) is a liquid that circulates through the car’s cooling system to stop the engine overheating. The expansion tank catches the inevitable overspill of this all-important fluid and recirculates it around the engine. The engine’s cooling system works by heating the coolant in the radiator to its required operating temperature. When this temperature is reached the liquid expands and is forced out of the radiator through the radiator cap. This is where the expansion tank comes into play. It stores the excess coolant to ensure that the necessary fluid level remains within the system. Are you frequently having to top up your coolant? You could have a leak somewhere in the cooling system. Check your expansion tank and if it’s leaking, find the replacement you need at Eurocarparts.com.