When the cover is closed, its sealing surface is firmly pressed against the edge of the opening of the expansion tank and if the safety valve is closed, then there is no pressure loss.
If, when heated, the pressure in the cooling system exceeds 1.2-1.5 bar, the safety valve opens and a certain amount of steam is ejected, the pressure is equalized.
When cooled, the liquid again decreases in volume, and a reduced pressure is formed in the cooling system. There is a second valve in the lid to counterbalance this reduced pressure. It opens at a reduced pressure of 0.06-1.0 bar so that outside air can enter the system.
Checking the safety valve
Those who do not want to suffer with the check, in a suspicious case, simply have to replace the relatively inexpensive screw cap. The workshop checks it with a pressure gauge:
Increase pressure.
At a pressure of 1.2-1.5 bar the valve should open.
You can only roughly determine whether this valve is working.
With the screw cap removed, firmly squeeze the thick tube of the cooling system.
Put on and screw on the cover.
Release the hose.
If the compressed hose rounds up again, then the valve is most likely OK.
If the tubes of the cooling system look flat and squeezed in the morning before the first start, then the vacuum valve is probably defective.
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