WARNING: This page is about a different car, the 2013 BMW 550i GT, 2012 BMW 550i GT, 2011 BMW 550i GT, and 2010 BMW 550i GT. 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]