WARNING: This page is about a different car, the 2011 BMW 750xi, 2011 BMW 750Lxi, 2010 BMW 750xi, and 2010 BMW 750Lxi. However, it is still accessible from the selected car via links, so may be relevant.
0491385 170001 Adapter MW
NOTE:
(Adapter) For checking the pressure relief valve in the radiator cap / Series: all except E28 [Discontinued]