SQ5 Boost Issues After Shop Work Kills Performance

Had similar symptoms on a 2017 S4 with the 3.0T engine. The main issue turned out to be a defective vacuum switching valve causing boost pressure inconsistencies. The pressure sensor readings were erratic, leading to significant turbo lag and poor throttle response. Initially, the symptoms matched yours, sluggish acceleration, increased fuel consumption, and intermittent check engine warnings. The pressure drop was most noticeable during acceleration, especially between 2000-3000 RPM. The root cause was the bypass valve not functioning correctly, leading to compressor surge under load. This affected the entire boost control system. The fix required replacing the vacuum switching valve assembly and recalibrating the pressure sensor system. This is typically a serious issue that should be addressed promptly, as continued operation can potentially damage the turbocharger system. The repair itself is straightforward for a qualified technician with proper diagnostic tools. Would recommend having a proper boost leak test performed to confirm the diagnosis, as similar symptoms can also stem from damaged boost piping or failing turbo seals.

Thanks for sharing that, it sounds exactly like what I am experiencing with my SQ5. The engine performance issues and check engine warnings match up perfectly, especially that lack of power in the mid-range you mentioned. Did you remember roughly how much the repair cost for the vacuum switching valve and recalibration? Also curious if youve had any boost solenoid or bypass valve issues since getting it fixed? My turbo lag seems to be getting worse and I want to budget appropriately before taking it to the shop. The compressor surge sound is particularly concerning.

Thanks for asking about the repair costs. After getting the work done, the boost pressure control fault was completely resolved. The total came to 140 Euro for replacing the faulty vacuum switching valve, which was actually less than expected. Engine performance returned to normal immediately after the fix, and the bypass valve has been functioning perfectly since then. No more turbo lag or compressor surge issues to report. The intake manifold pressure readings are now consistently stable, especially in that critical 2000-3000 RPM range where the problems were most noticeable before. Have not experienced any boost controller related issues in the time since the repair. The technician also checked the surrounding components during the service to ensure there weren't any other potential failure points in the system. If your symptoms are similar, this repair should resolve the performance issues youre experiencing. Just make sure the shop has experience with these specific boost pressure control systems, as proper diagnosis and calibration is essential for a lasting fix.

Thanks for the helpful advice. Finally got the car diagnosed and fixed last week. The symptoms perfectly matched what was discussed, it turned out to be exactly the vacuum switching valve issue causing the boost pressure problems. The shop found a small vacuum leak near the intake manifold that was throwing off the whole boost control system. The boost solenoid was actually fine, but the faulty valve was causing erratic pressure readings and terrible turbo lag. Total repair came to 165 Euro including labor and parts. The car runs like new now, no more sluggish acceleration or check engine lights. Fuel economy is back to normal and the boost comes on smooth and strong. Really glad I got it sorted before any serious damage occurred to the turbocharger. Lesson learned about getting proper diagnosis from qualified techs who know these systems well. The previous shop missed these issues completely. Thanks again for pointing me in the right direction with this.