Coolant Recovery Tank
An unpressurized reservoir that stores overflow coolant as the engine warms up and returns it as it cools.
A coolant recovery tank (also called an overflow reservoir) is a plastic container connected to the radiator filler neck by a small hose. As engine coolant warms up, it expands; when pressure exceeds the radiator cap limit, the cap opens and routes the excess hot coolant into the recovery tank.
When the engine cools down, the coolant contracts, creating a vacuum in the radiator. This vacuum draws the stored coolant from the recovery tank back into the radiator, keeping the cooling system completely full and free of air pockets.
The coolant recovery tank is typically unpressurized and has 'hot' and 'cold' level marks to check coolant levels easily. If the tank is cracked, or if the connection hose is leaking, coolant will escape, leading to air entering the system and eventual engine overheating.