BMW 8 Diesel Engine Hissing and Power Loss Mystery

Had the same problem with my 2016 BMW 740d. The symptoms matched exactly what you're describing, power loss, rough running, hissing, and increased fuel consumption. Last month at the garage, they confirmed it was indeed the intake manifold control flap. Mine was stuck in one position, causing improper air distribution. The actuator motor had failed, and one of the flaps was actually cracked. They had to replace the entire intake manifold assembly because of the integrated design. It's not a DIY job, requires specialized diagnostic tools and precise calibration after installation. The repair took about 6 hours. It's definitely a serious issue that needs attention soon. Continuing to drive with this problem can lead to more engine damage and worse fuel economy. Get it checked by a BMW specialist, regular mechanics sometimes miss the underlying cause.

Thanks a lot for sharing your experience! Really helpful to know I'm not alone with this. Just curious, do you remember roughly how much you ended up paying for the whole repair? Also, has everything been running smooth since the fix, or have you noticed any other issues pop up?

Hey again! Glad I could help out with my previous experience. Yeah, I can confirm that after getting it fixed for 465 € (the switch for the intake manifold control was defective), my 740d has been running like a dream. No more power issues, weird noises, or excessive fuel consumption. It's honestly like having a different car, smooth and powerful just like it should be. The mechanic really knew what they were doing, and it turned out to be exactly what I suspected with the intake manifold flap control. Haven't had a single related issue since the repair, which was about a month ago now. The engine's performing perfectly, and those annoying symptoms are completely gone. If you're experiencing the same problems I did, I'd definitely recommend getting it sorted soon. The price I paid was worth every penny considering how much better the car runs now. Plus, you'll probably save that money back in fuel costs pretty quickly!

Thanks for all the info! Actually, I bit the bullet and took it to a different mechanic yesterday. You were spot on, it was the intake manifold flap control. Cost me a bit more at 520 €, but honestly, what a difference! Car's running like new again. No more hissing, got all its power back, and fuel consumption's back to normal. The new mechanic was really thorough, showed me the faulty parts and explained everything. Funny thing is, they finished the whole job in about 4 hours, way quicker than my last mechanic would've taken. Guess finding the right mechanic makes all the difference. Really appreciate everyone's help here. Sometimes you just need that push to get things sorted properly rather than worrying about it!