This is a known issue. There was a part change on the cap after '10 I believe. One key thing is to keep the cap rubber ring clean as well as the mating surface. Make sure neither surface is dry, or the rubber will bunch up. You can also gently bend the fasteners to make a tighter seal.
08 E90M3 6MT Alpinweiss/Schwartz/BMW Pedals/Dinan Exhaust