Radiator Caps: Description
The radiator cap consists of a pressure valve and a vacuum valve. The cap has several different functions: It prevents coolant loss when vehicle is in motion; prevents impurities from entering cooling system minimizing corrosion; allows atmospheric pressure to eliminate the vacuum that occurs in system during cooldown; and raises coolant boiling point approximately 2°F per psi of pressure by maintaining a constant cooling system pressure. See COOLING SYSTEM SPECIFICATIONS article in this section.
NOTE:
Cap should maintain constant cooling pressure in system.