Non-Pressure System
WARNING: This page is about a different car, the 2014 Dodge Journey. However, it is still accessible from the selected car via links, so may be relevant.
This system works in conjunction with the pressure cap to utilize thermal expansion and contraction of the coolant to keep the coolant free of trapped air. It provides a volume for expansion and contraction, provides a convenient and safe method for checking coolant level, and adjusting level at atmospheric pressure without removing the pressure cap. It also provides some reserve coolant to cover minor leaks and evaporation or boiling losses.