Radiator Cap


One inexpensive item that a vehicle often needs is a new radiator cap. The rubber seal on the pressure cap can become worn or damaged with age, the pressure spring can rust, or the radiator filler neck seat can be damaged. A damaged cap can allow pressure to escape from the radiator. If pressure is not maintained on the coolant, its boiling point can drop from 40° to 50°F. This causes the radiator to boil over in hot weather or when driving in high altitude.

Several things are checked on a radiator cap:

Radiator cap inspection. Courtesy of Ford Motor Company.
  • Check to see that the rubber seal is not torn or imprinted so that it no longer provides an effective seal.
  • Inspect the pressure valve spring for rust damage and freedom of movement.
  • Inspect the vacuum valve to see that it is not stuck, broken, or plugged, and that the seal is not damaged. Vacuum valves can be of two types: spring loaded and ones with no spring.
  • The cap should have the correct pressure specified by the manufacturer.