Some cooling systems employ the use of a translucent plastic container. This plastic container is linked to the overflow tube coming from the radiator. It gives off additional storage space for the coolant when it spreads out, and is named as the overflow, or expansion tank. It is otherwise known as the overflow canister and coolant reservoir.

Damaged expansion tanks are not much of a problem because Replacement expansion tanks are readily available. Now, this is how the expansion tank serves it job. The coolant inside the tank expands when the engine heats up. Without the presence of the expansion tank, the coolant would pour out of the overflow tube away from the cooling system and run down the street.

The expansion tank is the place where the coolant flows into, to prevent spills and messes. Because a vacuum is produced in the cooling system as the engine cools, the vacuum brings about some of the coolant in the expansion tube to be siphoned back into the system. Since the cooling system containing an expansion tank is practically a locked system, the coolant can circulate between the expansion tank and the system as it swells up and narrows down. Thus, no coolant is gone if the system is operating correctly. One more function of the expansion tank is to eliminate air bubbles from the cooling system. Coolant lacking air bubbles is a lot more efficient since it soaks up heat quicker. One advantage of the expansion tank is that as the level of coolant held in it increases and drops, the radiator is always packed full. Older vehicles can be effortlessly fitted with expansion tanks by merely fastening the tank close to the radiator, linking it to the overflow tube, and changing the radiator cap.