Gas Engine
WARNING: This page is about a different car, the 2012 Jeep Patriot and 2012 Jeep Compass. However, it is still accessible from the selected car via links, so may be relevant.
The system works in conjunction with the cooling system pressure cap to utilize thermal expansion and contraction of the coolant to keep the coolant free of trapped air. The system provides space for expansion and contraction. Also, the system provides a convenient and safe method for checking and adjusting the coolant level at atmospheric pressure without removing the pressure cap. It also provides some reserve coolant to compensate for minor leaks and evaporation or boiling losses.