Radiator cap function test

The radiator cap is a very important part of the engine’s cooling system, yet it is often overlooked when troubleshooting the system. If an individual expansion tank is installed, which is mostly the case today, the expansion tank cap is equally important. The radiator cap and expansion tank cap make sure the filling hole of the radiator and expansion tank are sealed off gastight. But they do a lot more than just that.

Caps are designed to ensure that the prescribed pressure on the cooling system is maintained at all times during operation by allowing air to escape in case of overpressure. They should therefore be considered “the safety valve” of the cooling system.

A damaged cap cannot effectively seal off the system nor keep it properly pressurised which will compromise the proper functioning of the cooling system. Even a new hose can go flat with a bad cap.
So, when performing routine cooling system maintenance or repairs, make sure to inspect the caps and replace any that are defective.


Follow these steps to help you identify a faulty cap:

Cap function check

  • Look up the pressure rating specified for the vehicle and compare it with the pressure rating printed on the cap. If incorrect, replace the cap.
  • Visually inspect the condition of the main seal, pressure seal and return seal. Replace the cap if the seals are cracked, hardened or damaged.
  • Both the pressure relief and vacuum relief valve must be easy to lift and spring back after release. If not, replace the cap.
  • There must be resistance on the spring. If there is none, the spring has lost its force and the cap needs to be replaced.
  • Pressure test the cap with a professional pressure testing device (follow manufacturer’s instruction). If the cap fails to hold the rated pressure, replace the cap.
  • Always check the radiator/expansion tank when replacing a faulty cap. It may have caused a dangerous build-up of overpressure causing bursts or cracks which ultimately lead to leakage.