VM Part Expansion tank

The expansion tank retains coolant in a reservoir that's been forced out of the car's radiatior under pressure. As you run your car, the temprerature of the coolant circulating through the water jackets naturally increases causing increasing pressure in the cooling system. When the pressure builds to a high enough point, the radiatior cap allows that pressure to escape into the coolant reservoir through a small hose, called an overflow hose connected by the neck of the radiator to the expansion tank. Most of the coolant that is forced into the expansion tank returns to the radiator naturally after a few hours or when the engine is cold.

If you had no expansion tank on your car, then the coolant that was released under pressure would spill out onto the road.If you keep having to fill the expansion tank with large amounts of coolant every day, more than a cupfull, then there is likely a small leak somewhere else in the system. That could also be due to a faulty radiator cap, a faulty radiator, water pump or hoses or in the worst case scenario-a blown head gasket or cracked block. If in doubt, have the radiator pressure tested to determine where the leak is coming from.