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The expansion tank is the reservoir that holds excess coolant until needed by the engine and allows for the expansion and contraction of coolant as it heats up and cools down. Expansion tanks are often the highest point in the cooling system of your Saab, whether attached to the radiator or set apart in the engine compartment and connected to the cooling system using a hose. This placement allows any air trapped in the cooling system to be pushed into the expansion tank, where it can escape, and allows coolant to be added to the cooling system without allowing air to enter.