BMW 4 Diesel: Black Smoke and Glow Plug Warning Signs

Had the same drama with my 2020 BMW 3 diesel at 68,000 km. Your symptoms sound exactly like what I experienced, glow plug warning, black smoke, cold start issues. Found out the wiring harness connecting to the glow plug control module was corroded and damaged. This caused intermittent connections and messed up the preheating sequence. The connector plugs were badly oxidized too. Had to get the entire glow plug control module and wiring harness replaced, plus new glow plugs since two were faulty when tested. Not a quick fix, car was at the garage for three days. Pretty serious issue since it affects starting and emissions. Some mechanics might just replace glow plugs, but get them to check the control module connections first. The black smoke usually means incomplete combustion from poor preheating. Don't wait too long, running like this can damage your engine.

Thanks so much for sharing your experience, it's really helpful to hear from someone who went through the same thing! My symptoms match yours exactly. Would you mind telling me roughly what you ended up paying for the full repair job? Also curious if you've had any other issues pop up since getting it fixed? My wallet's already crying thinking about the bill, but better to fix it properly than risk engine damage.

Hey there! Just wanted to update you on my repair experience. Had my BMW sorted last week and yes, it was exactly as I suspected, the preheating control unit was acting up. Only paid 95 € to fix a dodgy cable connection, which was way less than I initially feared! The good news is everything's running perfectly now, no more glow plug warning lights, black smoke's gone, and she starts like a dream even on cold mornings. Such a relief it wasn't anything more serious. Haven't had a single problem since the repair, honestly. Much cheaper than my original repair where they changed out the whole unit and glow plugs. Sometimes it's just the simple things, you know? My mechanic said these connection issues are pretty common on these models. If you're still having trouble, definitely get them to check the wiring first before replacing any major parts. Could save you a bunch of cash. Let me know how you get on with yours!

What a relief to hear back from you! I actually just got my car back from the shop yesterday. You were spot on about checking the wiring first, turns out I had a similar issue but needed a bit more work. The mechanic found two corroded connectors and a faulty control module that needed replacing. The total bill came to around 350, more than your repair but still way better than I'd feared! No more black smoke, starting issues, or warning lights. The engine runs so much smoother now, I can't believe I waited this long to get it fixed. The mechanic showed me the old parts and explained everything clearly, way better experience than my last one. Really glad I asked here first instead of just replacing everything blindly. Thanks again for your help, you saved me from potentially spending a lot more than necessary! Time to enjoy my smoothly running BMW again. All the best!