RS6 Turbo Lag and Performance Loss with CEL

Had similar symptoms on my RS4 (2016) last summer. The turbo lag and poor engine performance were caused by a faulty boost solenoid. The intake manifold pressure sensor was also showing incorrect readings, which triggered the check engine light. The root cause was a defective vacuum switching valve, causing boost pressure inconsistencies. This affected both turbos, leading to significant power loss and increased fuel consumption. While not immediately dangerous, running the engine with this issue can potentially damage other components if left unaddressed. The repair involved: Testing boost pressure at various RPM ranges, Replacing the vacuum switching valve, Recalibrating the pressure sensor, Full diagnostic system reset Total repair time was about 4 hours. This is a known issue that requires proper diagnostic equipment and experience with performance engines. The symptoms you describe match exactly what I experienced, especially the turbo lag and poor throttle response. Would recommend going to a shop specializing in performance vehicles, particularly those familiar with twin-turbo setups. This isnt a repair for a general service center.

Thanks for sharing your experience with the vacuum switching valve issue. The symptoms match my current situation perfectly, especially the bypass valve behavior and compressor surge problems. I was wondering about the total cost of your repair? Also, has everything been running smoothly since the fix, or have you noticed any other boost-related problems? My engine performance is really suffering and I want to make sure I address all potential issues in one go. The pressure sensor readings on my car are all over the place, so I might need a similar fix. Planning to take it to a specialist but want to have a rough idea of the costs involved.

Had the repair done last week actually. Since fixing the boost controller and vacuum switching valve issues, the car runs perfectly, no more pressure drop or compressor surge problems. The specialist diagnosed it spot on, just like what happened with my previous RS4. Total cost came to 140 Euro for replacing the defective vacuum switching valve. Money well spent considering how smoothly everything works now. The boost pressure stays consistent across all RPM ranges, and fuel economy is back to normal. These high-performance engines are sensitive to vacuum leaks and boost pressure irregularities. Getting the bypass valve system properly calibrated made a huge difference. No warning lights or error codes since the repair, and throttle response is crisp again. Worth noting that proper diagnostic equipment is essential for these issues. A general mechanic might miss the subtle pressure variations that specialist tools can detect.

Thanks for the input. Finally got my car back from the specialist shop yesterday. Turned out it was exactly what you described, faulty boost solenoid and pressure sensor issues causing the turbo lag. The mechanic found a crack in the bypass valve housing that was causing inconsistent boost pressure. They replaced the whole assembly along with the vacuum switching valve. Also recalibrated the pressure sensor system since readings were way off. Total cost came to 680 Euro including parts and labor. More than I hoped but worth every cent. The turbo response is back to normal and fuel consumption dropped back to regular levels. No more check engine light either. Much better experience than my last mechanic visit. The specialist really knew what they were looking for and had the right diagnostic tools to properly test the boost pressure system. Car pulls like new again with zero turbo lag. Really appreciate the advice that helped me identify the issue before going in. Saved me from getting unnecessary repairs or wrong parts replaced.