BMW 6 Diesel Losing Power with Whistling: Boost Issues?

Hey there! I had nearly identical issues with my 2008 BMW 535d (168k km). That whistling sound and power loss got me worried too. Turned out it was indeed the electrical bypass valve acting up, just like you're suspecting. Had it fixed last month at the garage. The mechanic showed me the old valve, it was completely stuck and corroded. They had to replace the entire valve assembly and reprogram the ECU. Job took about 4 hours. It wasn't a super serious issue, but leaving it unfixed would've eventually damaged the turbo. Since the repair, my car's running like new, no more whistling, power's back, and fuel consumption's normal again. Those symptoms you're describing are textbook bypass valve failure. Worth getting it checked soon, especially with that engine light on. Just make sure you find a garage that specializes in BMW diesels, these systems can be tricky.

Thanks for sharing your experience! That's exactly what I needed to hear. Mine does sound just like what you went through with your 535d. I'm feeling more confident now about what the issue might be. Quick question, do you remember roughly how much you paid for the whole job? Also, has everything been running smoothly since the repair, or have you noticed any other issues pop up?

Hey again! Glad I could help! Just wanted to update you, since my last post, everything's been running absolutely perfect with my 535d. The repair cost me 250 € total, which covered both the new electrical bypass valve and the labor. Looking back, it was money well spent. And to answer your question, no other issues have popped up since then. The whistling's completely gone, power delivery is smooth, and fuel economy is back to normal. The engine light hasn't come back either. The mechanic did a great job diagnosing and fixing the boost pressure actuator fault. You know what's funny? I was also worried it might be something more serious (and expensive!) at first, but it turned out to be a pretty straightforward fix. If your symptoms are like what I had, you're probably looking at the same repair. Just make sure you get a good BMW diesel specialist to handle it, it makes all the difference. Feel free to ask if you need any other details about the repair. Really happy with how my car's performing now!

Thanks so much for all the info! You've really helped me feel better about this whole situation. I actually took your advice and found a BMW specialist mechanic yesterday. You were spot on, it was the boost pressure actuator! The repair cost was slightly higher at 320 €, but honestly, it was worth every penny. The mechanic was fantastic, showed me the faulty part and explained everything clearly. What a difference from my last garage experience! No more whistling, the engine's got its power back, and it's running smoother than ever. Funny how we worry ourselves sick over these things, right? I was imagining all sorts of expensive repairs, but it turned out to be pretty straightforward, just like you said. The mechanic even cleaned up some minor issues while he was at it, no extra charge. Really appreciate you taking the time to share your experience. You saved me a lot of stress and probably some money too by pointing me in the right direction. Car's purring like a kitten now!