BMW 4 Diesel Glow Control Issues, Cold Start Problems

Hey there! I had a similar issue with my 2014 BMW 3 diesel. I've got some experience working on cars, but nothing too fancy. Had almost identical symptoms on mine, preheating light stuck on, rough cold starts, and those annoying glow plug codes. Took it to my regular workshop and they found it was just a loose connection in the wiring harness to the glow control module. Fixed it up, including some new connectors, and it cost me around 95 Euro. The smoke and cold start issues disappeared completely after that. Before jumping to conclusions about the glow control unit itself, I'd definitely get those connections checked first, could save you some money! Mind sharing what error codes you're getting specifically? Also, have you noticed if the symptoms get worse in colder weather? I know the 4 series pretty well since it shares a lot with the 3 series. Let me know those details and I might be able to help more.

Thanks for sharing your experience! My car's got 170337 KM on it, and I had the last service about 2 months ago. Regarding the error codes, I'm getting P0380 and P0670. The symptoms definitely get worse when it's cold outside. I checked the connections myself but couldn't spot anything obvious. The interesting thing is that it started happening gradually, first just occasional rough starts, then the constant preheating light, and now the smoke. Good to know about your wiring harness fix though, that's definitely something I'll have checked before replacing the whole glow control unit. Did your mechanic have to remove a lot of parts to access the wiring?

Hey again! Thanks for those details about your BMW. Yeah, my 3 series had about 160,000 KM when I had similar issues, so we're in the same ballpark mileage-wise. Those error codes you mentioned, P0380 and P0670, were exactly what I got too! Initially thought it was just the glow control unit, but as I mentioned before, it turned out to be the wiring. Regarding access to the wiring, honestly, it wasn't as bad as you might think. My mechanic did have to remove the engine cover and a few other bits to get proper access, but it wasn't a major teardown job. Since you mentioned it started gradually, that's another reason why I'm leaning towards it being a wiring issue like mine was. These problems tend to get worse over time as the connections deteriorate further. The cold weather basically exposes the weakness in the system, that's why you're seeing it more now. One thing I forgot to mention in my first reply, after they fixed the wiring, they also cleaned the connections with electrical contact cleaner. Might be worth asking your mechanic about that too. Made a huge difference in the reliability of the system. Let me know how it goes when you get it checked out. Really curious if it turns out to be the same issue as mine!

Thanks for the detailed follow-up! That's really encouraging to hear that your repair wasn't too complicated. I've just booked my car in for next week, and I've specifically asked them to check the wiring and connections first before jumping to any conclusions about the glow control unit. The electrical contact cleaner tip is super helpful, I'll definitely mention that to them. It makes total sense about the gradual deterioration of the connections. Looking back, I probably should have investigated when the cold starts first began, but you know how it goes, you always think it'll sort itself out! At least now I've got a better idea of what to expect. I'll update you once I get it sorted. Really appreciate all the help and sharing your experience, saved me potentially spending way more than necessary on parts I might not even need!