The sealing surface, when the cap is screwed on, must be firmly pressed against the edge of the opening of the expansion tank. Pressurized steam must not pass through.
If, when heated, the pressure in the cooling system exceeds 1.2-1.5 bar, the safety valve opens. In this case, part of the steam is vented to equalize the pressure.
When cooled, the coolant again decreases in volume and a vacuum occurs in the cooling system. To equalize the pressure in the cover there is a second valve. It opens at a pressure of 0.06-0.1 bar and outside air can enter the expansion tank.
Checking the safety valve
Create pressure with a pump.
The valve should open at a pressure of 1.2-1.5 bar.
Checking the vacuum valve
The serviceability of the vacuum valve can only be verified by testing.
With the expansion tank cap removed, squeeze the thick coolant hose. Put the lid on and screw tight.
Release the hose.
If the squashed hose straightens out again, maybe. that the valve is defective. If the coolant hoses are compressed in the morning before starting the engine, then the vacuum valve is probably not working.
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