Checking vacuum valve in cap
WARNING: This page does not describe the selected car, but rather 15 other vehicles, including the 2005 BMW 330xi, 2005 BMW 330i, 2005 BMW 330Ci, 2004 BMW 330xi, and 2004 BMW 330i. However, it is still accessible from the selected car via links, so may be relevant.
Screw cap (1) onto special tool 17 0 007 and connect to vacuum hose of vacuum pump.
Sourcing reference: BMW Parts Service.
Feed vacuum pump with battery positive voltage.
Slowly apply partial vacuum to cap.
Compare opening pressure of vacuum valve.