Wet the valve with soapy water. If an air bubble forms, tighten the valve "3" tightly using the inverted dust cap "2".
Attention: The valve can only be tightened with the metal cap "2". This cap is located on the fuel tank side. 1 - rubber protective cap; 4 - valve body.
Check the valve again. If air bubbles still form and the valve does not tighten further, it must be replaced (workshop work).
Be sure to reattach the dust cap.
(Information taken from this resource BMWMAN.ru)