Checking pressure relief valve in cap
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2010 BMW 335xi, 2010 BMW 335i, 2009 BMW 335xi, 2009 BMW 335i, and 2008 BMW 335xi. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
While the vehicle is driven at high outside temperatures, the design may cause the pressure relief valve in the cap to open slightly and air together with dissolved coolant to escape. This coolant vapour condenses on the surface of the expansion tank and leaves traces when the vehicle has cooled down. These traces do not indicate whether the cap is defective or not. When the vehicle has been parked up for an extended period of time, the residual escaping coolant can cause the pressure relief valve in the sealing cap to stick; therefore check the cap again 2 to 3 times.
Replace the cap only after you have checked three times and there is an incorrect opening pressure.