BMW M4 Vacuum Leak Causing Multiple Performance Issues

I experienced almost identical issues with my 2014 BMW M3 last winter. The symptoms matched yours exactly, hissing, rough acceleration, and poor fuel consumption. Turned out my intake boot had several small cracks and one major tear, causing a significant vacuum leak. Had it checked at the garage where they did a smoke test that revealed multiple air leaks in the intake system. The main culprit was the intake boot between the mass airflow sensor and throttle body. This wasn't just a minor issue, the leaks were causing unmetered air to enter the system, messing with the air-fuel ratio and engine performance. The fix required replacing the entire intake boot assembly and a few connecting hoses that had also deteriorated. After replacement, the car ran like new again, smooth acceleration, no hissing, and proper fuel economy. With these symptoms, I'd suggest telling your mechanic to specifically check the intake system with a smoke test. The high mileage on your M4 makes it a likely candidate for worn intake components.

Thanks for sharing that detailed experience with your M3! It sounds exactly like what I'm going through with mine. Really helpful to know about the intake boot being the culprit. Would you mind telling me roughly how much you ended up paying for the whole repair? Also, curious if you've had any similar issues pop up since getting it fixed?

Hey there! Happy to give you an update about my M3 repair. The fix worked like a charm, once they replaced that leaking intake hose, all those annoying problems disappeared. The total bill came to 100 € for the repair job, which honestly wasn't as bad as I expected. Since getting it sorted, my car's been running perfectly smooth, no more hissing noises, jerky acceleration, or fuel economy issues. It's been about 8 months now, and I haven't had any similar problems come back. Got to say, getting that smoke test done was the key, it showed exactly where the air was leaking in the intake system. My engine's performing just like it should now, and those failed emissions and check engine lights are a thing of the past. If you're still dealing with those symptoms, definitely get that intake system checked. The repair cost seems worth it considering how much better the car runs now. Trust me, you'll notice the difference right away once it's fixed!

Thanks everyone for your input! Just wanted to update you all on what happened. Finally got the courage to take my M4 to a new mechanic yesterday. You were spot on, it was exactly what you described with the intake system. They did the smoke test and found multiple leaks in the intake boot. The whole thing cost me a bit more than yours, around 150 for parts and labor, but honestly, it was totally worth it. The difference is like night and day! My car's running super smooth now, no more of that annoying hissing or jerking around when I accelerate. Even my fuel consumption is back to normal. Passed the emissions test too, which is a huge relief. Check engine light is finally off. Really glad I didn't put this off any longer. The new mechanic was actually great, showed me exactly where the problems were and explained everything clearly. Thanks again for helping me avoid another bad mechanic experience. Sometimes you just need that push to make the right decision!