Pressure Cap
WARNING: This page is about a different car, the 1988 Oldsmobile Toronado and 1988 Buick Riviera. However, it is still accessible from the selected car via links, so may be relevant.
Modern cooling systems use a closed system type cap. This system allows for coolant expansion during engine operation. As coolant expands and builds pressure, some coolant is permitted to bleed past the cap into the over flow tank. When the engine cools and coolant contracts, the cap allows the coolant in the over flow tank to siphon back into the system.
The pressure cap also increases pressure in the cooling system. The increased pressure raises the boiling point, one pound of pressure raises the boiling point approximately 10° F (12.2° C).