- The sealing surface of the screwed cap is firmly pressed against the edge of the opening of the expansion tank. The pressure can't go down
- If, during heating, the pressure in the cooling system exceeds 1.2-1.50 bar, the overpressure valve opens. A little steam can escape through it in order to balance the pressure.
- When cooled, the liquid contracts again. As a result, a certain reduced pressure develops in the cooling system. To equalize this reduced pressure, there is a second valve in the cover, through which atmospheric air can enter the system.
High pressure valve check
Those who do not want to spend time checking, in a suspicious case, simply change the relatively inexpensive cover. In the workshop, the locking cover is checked with a manometer:
- Increase the pressure with a pump.
- Between 1.2-1.50 bar the valve should open.
In 16V engines, a bolt with an internal hexagon is unscrewed to bleed air from the cooling system (arrow) on the connecting pipe.
In the locking lid (1) cooling system inserted pressure relief valve (2) and pressure relief valve (3). Information on timing valve checks can be found in the text on this page.
Checking the low pressure valve with improvised means
The correct operation of the reduced pressure valve can only be checked indirectly:
- After removing the locking cap, firmly pinch the thick water hose.
- Put on the lid and screw it on.
- Release the hose.
- If the squashed hose rounds again, the valve is good.
- If the coolant hoses are flat in the morning before starting the engine for the first time, then the low pressure valve is probably not in order.
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