S7 Turbo Issues: Power Loss and Engine Performance Drop

I had nearly identical symptoms with my 2015 S6, and being somewhat experienced with turbo-related issues, I can share what worked for me. The main problem turned out to be a defective vacuum switching valve affecting the engine performance. The pressure sensor was showing irregular readings, and the entire boost control system was compromised due to a vacuum leak between the valve and the turbocharger. After diagnosis, the workshop found that the bypass valve was not operating correctly due to the faulty vacuum switching valve. The repair involved: 1. Complete diagnostic scan 2. Pressure testing of the system 3. Replacement of the vacuum switching valve 4. System recalibration Total cost was 140 Euro including parts and labor. The difference in performance was immediately noticeable, proper boost response, smoother acceleration, and fuel consumption returned to normal. To provide more specific advice for your case, could you share: Current mileage, Any recent maintenance history, Whether you notice any unusual sounds when accelerating, If the issue appeared suddenly or developed gradually These details would help determine if your S7 is facing the same underlying issue or if there might be other factors involved.

My latest service was at 112000 KM and all service items were completed according to schedule. The turbo lag started appearing gradually over the last 2000 KM. The boost controller behavior you described matches my symptoms perfectly. No unusual sounds during acceleration, but the pressure drop is quite noticeable. Think I will follow your suggested diagnostic approach and have the vacuum system checked first. Appreciate the detailed breakdown of your repair process and costs. Really helpful to know what to expect when I take it to the shop.

Thanks for the additional info. Based on those mileage details and gradual onset of symptoms, it definitely sounds like the same vacuum leak issue I encountered. The progressive nature of the turbo lag and pressure drop is a classic sign of a deteriorating vacuum switching valve. On my S6, the problem also developed around the 110000-115000 KM mark. The gradual loss of engine performance started exactly like yours, subtle at first, then becoming more noticeable over a few thousand kilometers. One thing I forgot to mention in my first reply, while waiting for the repair appointment, I noticed the boost pressure would sometimes normalize temporarily after a cold start, then deteriorate as the engine warmed up. If you notice this pattern too, it further confirms the vacuum leak diagnosis. Since your service history is up to date, and there are no unusual sounds, I am even more confident this is the same issue. The bypass valve itself was actually fine in my case, it was just not getting proper vacuum signal due to the faulty switching valve. The repair solved all symptoms: Eliminated the turbo lag, Restored normal engine performance, Fixed the boost pressure control, Brought fuel consumption back to factory specs

I find your analysis of the vacuum switching valve very convincing, especially since my symptoms match yours so closely. The cold start behavior you mentioned is exactly what I am experiencing, temporary normal boost pressure that deteriorates as the engine warms up. This really confirms my suspicions about the vacuum leak being the root cause. Will book an appointment tomorrow to have the boost controller and vacuum system checked. The 140 Euro repair cost seems very reasonable considering the impact on engine performance and fuel consumption. Really glad I asked here before potentially replacing more expensive components unnecessarily. One last question, did you notice if your check engine light cleared immediately after the repair, or did it take a few drive cycles? Mine keeps coming back even after resetting it, but I assume this will resolve once the vacuum leak is fixed and proper boost pressure is restored. Thanks again for all the detailed information about your experience. It has saved me a lot of troubleshooting time and potentially unnecessary parts replacement.