As the engine heats up, coolant expands. The coolant expansion tank provides additional storage for the expanded coolant and is the reservoir where the driver tops off or adds coolant to the engine. The tank is normally clear in color so the driver can see the coolant level. On some vehicles, the driver may have to remove a cap to visually check the coolant level.
The expansion tank should be checked every other time the gas tank is filled. If the coolant level in the expansion tank consistently drops after topping off, there is probably a leak. A cooling system leak may be caused by a defective radiator, radiator cap, expansion tank, hose, or hose clamp.
The engine must be cool to replace the coolant expansion tank. The coolant is removed from the tank, as are the hoses going to the it. The old tank is removed from the vehicle, the new tank is installed, hoses are reinstalled, and the tank is refilled to its proper level.
The coolant expansion tank rarely needs to be replaced unless failure is common problem on that particular vehicle model.