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Bmw 3 Series Coolant Expansion Tanks Available Here. Your vehicle’s radiator scatters the heat that your coolant (antifreeze) has absorbed from the engine. It holds a large amount of water in tubes and tanks, one of which is the expansion or overflow tank. This plastic container is connected to the radiator by means of an overflow tube and provides extra storage space for the antifreeze when it expands and overflows the radiator. Without it the excess antifreeze would flow out onto the ground. As your engine cools off, a vacuum forms and the antifreeze is sucked back into the cooling system. The expansion tank also removes air bubbles from the system, keeping the radiator full at all times and making the antifreeze more efficient in absorbing engine heat. Common signs of a faulty coolant expansion tank include needing to constantly add coolant, finding coolant leaks, and the engine overheating.