Diesel BMW 8's Cold Start and Black Smoke Mystery

Had the same problems with my 2015 BMW 7 Series diesel. The symptoms you're describing sound exactly like what I experienced, glow plug system malfunction and the control module acting up. Initially thought it was just the glow plugs, but when my mechanic checked, turned out the wiring harness connecting to the control module was corroded and had loose connections. The black smoke was definitely a red flag, it's typically a sign of incomplete combustion. Got it fixed at another garage after my regular one couldn't diagnose it properly. They had to replace the entire glow plug control module and rewire the connections. It was quite a serious job since they had to access deep parts of the engine. The car runs perfectly now, no more hard starts, smoke's gone, and passed emissions with flying colors. Definitely recommend getting a second opinion from a BMW specialist who knows diesel systems well. Feel free to ask if you need more specific details about the repair process.

Thanks for sharing your experience, that's super helpful! Your situation sounds identical to what I'm dealing with. Really good to know it was fixable and your car's running well now. Just curious, do you remember roughly how much you ended up paying for the whole repair? And has everything stayed working properly since then, or have you noticed any other issues pop up?

Hey again! Just wanted to follow up on your cost question. In my case, it turned out way simpler (and cheaper!) than I initially feared. The mechanic found it was just the Preheating control unit playing up, specifically a faulty cable connection. Only cost me 95€ to fix, which was such a relief considering I was worried it might be something major! Everything's been running perfectly since the repair was done about three months ago. No more hard starts in the morning, and that annoying black smoke completely disappeared. The car's passing emissions tests with no problems now too. Really glad I got it sorted when I did, seems like these issues can get worse if you leave them too long. Trust me, if you're experiencing similar symptoms, it's worth getting it checked out. My BMW's running like a dream now! Let me know if you need any other details about the repair. Always happy to help another BMW owner out!

Thanks so much for all the details! That's actually really reassuring to hear it wasn't a huge expensive fix for you. I've been putting off getting it looked at because I was worried about getting hit with a massive repair bill. I actually took your advice and found a different mechanic who specializes in diesel BMWs. Turned out my issue was pretty similar, faulty wiring in the preheating system. The fix was straightforward and way cheaper than I expected. The mechanic also showed me exactly what was wrong, which I really appreciated after my last bad experience. Car's running perfectly now, no more morning start struggles or black smoke issues. Wish I'd asked for help sooner instead of stressing about it! Really appreciate you sharing your experience, it gave me the push I needed to get it sorted properly.