Expansion tanks provide a high point in the coolant system where a cushion of air can be present. When the coolant in the system builds up in temperature the fluid expands and creates pressure. When under pressure the cushion of air compresses and provides room for the expanding coolant. Expansion tanks generally should be mounted higher than the engine and radiator to ensure that the engine and radiator are completely filled and the tank itself should only be partially filled to allow for expansion.