M3 Boost Control Issues: Power Surge and Fuel Problems

Hey there! I've got some experience working on cars (some experience level), and this sounds very similar to what I had with my 2002 BMW M5. The symptoms you're describing match exactly what I experienced. After a few days of dealing with the same issues, uneven power delivery and increased fuel consumption, I took it to my trusted workshop. Turned out it was a faulty vacuum switching valve. Had it replaced and everything went back to normal. The total bill came to about 140 Euro, parts and labor included. Before I can give you more specific advice, could you share: Current mileage on your M3? Are you getting any specific error codes? Does the problem get worse when the engine is warm? By the way, these issues are somewhat common on the older M series BMWs, my had similar troubles with his E46.

Thanks for sharing your experience! My M3 has 194363 KM on the clock, and I just had it serviced last month. Definitely getting worse when the engine warms up, usually takes about 15 minutes of driving before it becomes really noticeable. Haven't pulled the error codes yet, planning to do that tomorrow. That repair cost sounds much better than what I was expecting! Did you notice any other symptoms before fixing the valve? I'm particularly wondering if you also had a rough idle or any weird noises.

Hey again! Glad to help further with your M3 situation. Yep, your mileage and symptoms are ringing more bells now, my M5 was around 180k when I had the same issue. Actually, now that you mention it, I did notice a slightly rough idle, especially when the engine was fully warm. There was also this faint whistling sound under acceleration, but it was pretty subtle, I almost missed it at first. The weird power delivery was definitely the most noticeable thing though. After getting the vacuum switching valve replaced, all those little quirks disappeared. The idle smoothed out, power delivery became consistent again, and my fuel consumption dropped back to normal. It's actually a pretty straightforward fix, took the workshop less than two hours. One thing I forgot to mention in my first reply, I initially thought it might be something more serious (and expensive!) like a turbo issue, but thankfully it wasn't. Definitely get those error codes checked, that'll help confirm if you're dealing with the same problem. My 's M3 had similar symptoms but turned out to be a different valve, so better to be sure. Keep me posted on those error codes when you get them, I'm curious to see if they match what I had. Pretty sure mine threw a P0034 code, but it's been a while.

Thanks so much for the detailed follow-up! Really appreciate you taking the time to share all those specifics. The whistling sound you mentioned, that's exactly what I've been hearing too but couldn't quite describe it! I'm feeling a lot more optimistic now that you've shared your experience. Just checked with my mechanic and he says he can check those error codes first thing tomorrow morning. I'll definitely let you know what codes come up. If it turns out to be that vacuum switching valve, I'll be pretty relieved, sounds way better than dealing with turbo issues! The symptoms match up so perfectly with what you described, especially that rough idle when warm. Just hoping it's as straightforward a fix as yours was. I might also have my take a look, he's been working on BMWs for years and might have some additional insight. Will update once I know more about those error codes. Thanks again for all the help, this community is always so helpful when it comes to sorting out these issues!