Checking pressure relief valve in sealing cap
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.
NOTE:
While the driving at high ambient temperatures, the design may cause the pressure relief valve in the sealing 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 toes do not indicate whether the sealing 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 sealing cap again 2 to 3 times.
Replace the sealing cap only after you have checked three times and there is an incorrect opening pressure.
Screw on sealing cap (1) to special tool 17 0 114.
Build up pressure with special tool (hand pump) 17 0 101; observe pressure gauge to ascertain when opening pressure is achieved.
Compare OPENING PRESSURE of pressure relief valve.