it seems to me that bmw have missed a fairy easy problem!
Theres an airlock in your system and when your engine gets hot the pressure build up is been expelled through the overflow pipe that runs down the side of your radiator therefore you wont notice it while your driving!
Easy solution take filler cap off and let engine run until it gets hot and any air trapped in it while rise to the top off the radiator like a bubbling sound make sure when its finished to top the radiator back up to the level using hot water not cold as this could crack the cylinder head!
ian mechanic
April 2009
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