BMW M3 Cold Start Nightmare: Rough Idle & High Fuel Use

Hey there! I had nearly identical issues with my 2012 BMW M5 last winter. The symptoms matched yours exactly, rough idle, hard starts, and terrible fuel economy. After diagnosing, it turned out two of my injectors were failing. When I took it to the garage, they ran injector pulse tests and found uneven spray patterns. The faulty injectors weren't atomizing fuel properly, especially during cold starts. This was causing the rough running and excessive fuel consumption you're describing. It was a serious job that took about a day and a half. They had to replace both injectors and recalibrate the engine management system. The fuel system needed a thorough cleaning too, as the bad injectors had left some deposits. Since the repair, the car starts instantly and runs smooth as silk. If your symptoms match what I experienced, I'd definitely get those injectors checked soon, running with faulty ones can damage your catalytic converter and other components. Hope this helps with your decision.

Thanks so much for sharing your experience! That's really helpful since the symptoms match exactly what I'm dealing with. Sounds like I should definitely get those injectors checked asap. Just wondering, do you remember roughly how much you ended up paying for the whole job? Also, has everything been running smoothly since the repair, or have you noticed any other issues pop up?

Hey again! Glad I could help! Yeah, I recently got mine fixed, your message actually reminded me to update you. The mechanic found exactly what I suspected: a faulty cold start injector. Total bill came to 515 € for replacing the injection valve. Money well spent, if you ask me! Since the repair, my M5's been running like a dream, starts up instantly, even on the coldest mornings. No more rough running, and my fuel consumption is back to normal. Haven't had a single problem in the months since the fix. If your symptoms are as similar as you say, I'd definitely recommend getting it checked out soon. The peace of mind alone is worth it, and you'll save money on fuel in the long run. Plus, you really don't want to risk damaging other parts by leaving it too long. Let me know how it goes when you get it sorted!

Thanks for the follow-up! I actually just got back from the mechanic today. You were spot on, it was definitely the injector causing all the trouble. My bill came in a bit higher at 680€ since they found some buildup they needed to clean out too, but totally worth every penny. The difference is incredible! My M3 purrs like a kitten now, starts right up without any fuss. Can't believe I waited so long to get it fixed. The mechanic was great too, walked me through everything they found and fixed. Way better experience than my last garage. Really appreciate you sharing your experience earlier. It gave me the confidence to tackle this issue head-on instead of putting it off. Think I've found my new go-to mechanic too! Thanks again for all the help!