Overflow Tank

The Overflow Tank (sometimes called the Overflow Bottle or Coolant Reservoir) is a tank that stores excess coolant, until the cooling system cools enough to retract and suck it back up.

Operation

As the pressure in the cooling system exceeds that at which the radiator cap is rated, the radiator cap bleeds the excess pressure off. From there, it runs into the overflow tank.
Once the cooling system has cooled down and the coolant has contracted, it creates a vacuum which then pulls the excess coolant up from the overflow tank and back into the radiator.

Location

The stock overflow tank is located on the driver's side (LHD), in front of the front wheel. In a stock twin turbo, it is directly behind the driver's side intercooler. The overflow tank uses a filler neck which leads from the tank to the small corner behind the driver's side headlight, where it is attached to the body of the car. The cap for the filler neck also includes a dip stick to indicate the level of fluid inside the overflow tank.

Because of its location directly behind the driver's side intercooler, some twin turbo owners feel the overflow tank impedes airflow through the intercooler and therefore relocate it to a place less obstructive. This is often difficult with the stock overflow tank, as it's quite long and flat. A common "upgrade" is to mount a smaller cylindrical overflow tank to the frame rail under the driver's side headlight, in front of the radiator.