My 2008 BMW 335xi backfires when it is cold. There appears to be nothing else wrong with the car and the dealer’s service department has told me on various visits: we can’t reproduce it; or it is “normal”; it’s a turbo and runs rich so typical; or this other new BMW on the lot does the same thing so it’s expected. The backfire only happens when the engine is cold, but EVERY time it is cold. I just had the exhaust replaced because the backfire damaged it. The service department is sick of seeing me and does not have an answer. My car is still under warranty.