BMW Overheating Yet Cold: Thermostat Troubles?

Hi there! I have some experience with cars and had a very similar issue with my 2018 BMW 3 Series diesel (320d). The symptoms were exactly like yours, slow warm-up, overheating, poor fuel economy, and inconsistent cabin heating. Took it to my regular workshop and it turned out to be a faulty map thermostat. The repair cost me around 350 Euro, but it solved all those issues completely. The mechanic explained that these electronic thermostats can fail and cause these exact symptoms. Could you share which specific 1 Series model you have (116d, 118d, etc.)? This would help me tell you if it's the same component as mine. Also, how many miles/kilometers has your car done? This info could be useful to confirm if we're dealing with the same problem. Let me know these details and I can share more specific info about my repair experience.

Thanks for sharing your experience! I drive a 118d with 158090 KM on the clock. Just had my regular service last month and everything else seemed fine then. Your description matches my issues perfectly, especially the slow warm-up and poor fuel economy part. Really good to know it was just the thermostat in your case. Did you notice any warning signs before it completely failed? Also, how long did the repair take? I'm hoping to get this sorted quickly since I use my car daily.

Hey again! Yeah, my 320d had about 145000 KM when this happened, so pretty similar mileage to yours. With the BMW diesel engines from this generation, the thermostat issues are pretty common actually. Looking back, there were some warning signs I initially ignored. First noticed the longer warm-up times, especially in the morning, took like 10-15 minutes instead of the usual 5. Then the fuel consumption started creeping up, probably getting about 2L/100km more than usual. The really noticeable thing was how the temperature gauge would fluctuate a bit during longer drives. The repair itself wasn't too time-consuming, my mechanic had it done in about 3 hours. They had to drain the coolant, replace the thermostat unit, and then refill and bleed the cooling system. The part itself was genuine BMW and came with a warranty. Since the fix, everything's been running perfectly, normal warm-up times, stable temperature, and fuel economy is back to normal. Definitely worth getting it sorted sooner rather than later. These symptoms won't fix themselves, and running the engine outside its optimal temperature range isn't great for it long-term. Let me know how it goes when you get it checked out!

Thanks so much for all this detailed info! This is super helpful, especially knowing the repair time and that it's a common issue with these engines. The warning signs you mentioned are exactly what I'm experiencing, the slow warm-up and extra fuel consumption are spot on. Think I'll book it in for next week since it only takes a few hours to fix. Really don't want to risk any engine damage by putting it off. You've definitely saved me some stress and probably some money too, at least now I can go to the mechanic with a pretty good idea of what's wrong instead of them having to spend time diagnosing it. Will keep an eye on the temperature gauge on my drives this week, but sounds like I should just get that thermostat replaced asap. Thanks again for sharing your experience, it's been really helpful!