Engine coolant expansion tank. Cap not included. Your vehicle’s engine coolant recovery tank, or expansion tank, stores excess pressure and coolant as your radiator heats up. The reservoir helps keep your radiator full at all times and makes the radiator more efficient and effective. If your coolant is low, it may indicate a leak. Shops and dealers can use a pressure tester to check the system for smaller hard to find leaks. If your recovery tank is damaged, replace it immediately to avoid a catastrophic cooling system failure.