Radiator Cap: Description and Operation

NOTE: Numbers in parentheses ( ) indicate a component or tool in the associated illustration.

TASK
- In the engine-off heating phase (after engine has been switched off) the pressure in the cooling system may rise.
- An additional pressure stage has been incorporated in the cap of the expansion reservoir to prevent any coolant flowing out

OPERATION
- The cap has two opening pressures.

Stage 1
- If the pressure in the cooling system rises above the first pressure stage, the valve (3) opens and the pressure flows into the top part of the cap (1)

Stage 2
- If the pressure continues to rise, the valve (4) of stage 2 opens and the pressure flows through several vent drillings (a) arranged at the cap of the expansion reservoir (1) along the annular passage (b) into the outlet (c) and along a hose extension to atmosphere.
- The overflow reservoir is no longer installed with the introduction of the 2-stage cap.