Pressure Cap

Modern cooling systems use a closed system type cap. This system allows for coolant to expand and build pressure, some coolant is permitted to bleed past the cap into the overflow tank. When the engine cools and coolant contracts, the cap allows the coolant in the overflow 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 increases the boiling point approximately 3°F (1.66°C).