Coolant Expansion Tank
A reservoir that holds excess coolant as it expands with heat, and vents or returns it to maintain proper cooling system pressure.
The coolant expansion tank, also referred to as a coolant overflow tank or recovery reservoir, is a plastic container connected to the radiator. Its primary purpose is to act as a storage reservoir for engine coolant. As the engine warms up, the coolant expands, creating pressure in the radiator. When the pressure exceeds the rating of the radiator cap, the cap valve opens and allows the excess hot coolant to flow into the expansion tank.
As the engine cools down and the coolant contracts, a vacuum is created in the radiator, which draws the coolant back from the expansion tank. This closed-loop system prevents air from entering the cooling system, which could otherwise cause localized hot spots, corrosion, and air pockets that disrupt coolant flow.
Modern expansion tanks are often pressurized and fitted with their own pressure cap. Because they are made of plastic and subject to constant heat cycles, expansion tanks can become brittle, crack, and leak. Symptoms of a bad expansion tank include visible cracks, leaking coolant pools under the tank, or a low coolant level warning on the dashboard. Replacing a cracked tank is a straightforward repair that prevents coolant loss and overheating.