A coolant expansion tank is a common component found in most automotive applications. They are typically connected by a tube to the radiator filler neck, and can serve as a place for excess coolant to be captured when the pressure release valve in the radiator cap gives way. In a properly operating cooling system, the coolant will then be siphoned back into the radiator as the temperature drops. Certain applications can also use the coolant expansion tank as the fill point, in which case it serves as a pressurized part of the system. Many older cars and trucks lacked this type of tank, and any excess coolant would simply drain out onto the ground.
One common name for the coolant expansion tank is overflow tank, which describes its general purpose. As coolant within the radiator, block, and hoses heats up, it expands. This typically creates pressure within the system, and many cooling systems will operate under 10psi (69kPa) or more. If the system was completely sealed, the growing pressure could cause radiator hoses to blow out or damage other components. To help avoid this situation, many radiator caps are equipped with a valve that can release at the correct pressure, allowing some coolant to drain into an overflow tank.
Many older vehicles lacked expansion tanks. Some expansion could occur within the radiator itself, as it was common to leave an inch or more of air at the top of the radiator tank. When the pressure in the system became too great, first the air would vent through the radiator cap, and then coolant would drain out onto the ground. Certain vehicles that lacked a coolant expansion tank had room in the engine compartment to install an aftermarket unit, while some people secured an old bottle of some kind next to the radiator to catch the excess fluid.
Some newer vehicles may lack a filler neck on the radiator. One place these vehicles may locate the fill point is on the engine near the coolant inlet, though others will use the plastic expansion tank instead. These pressurized vessels are sometimes prone to cracking, and will often become very contaminated with sludge and residue. It is usually a good idea to clean out any overflow tank when flushing a cooling system, though it can become even more important when the expansion tank is an integral part of the cooling system. Many of these tanks contain a large number of baffles and chambers, which may result in difficulty when trying to clean out built up sludge.