RSQ3 Turbo Issues: Power Loss and High Fuel Usage

I experienced something very similar with my 2018 RS3. Initially noticed a significant engine performance drop and the same symptoms you describe, which turned out to be a vacuum leak caused by a defective vacuum switching valve. The symptoms match exactly what you are experiencing, poor throttle response, increased fuel consumption, and that annoying CEL. After some diagnostic work, the workshop found that the intake manifold pressure was fluctuating abnormally, leading to compressor surge issues. The repair involved replacing the vacuum switching valve and checking all connected hoses. Total cost was 140 Euro including labor. Since the fix, the engine performance returned to normal and fuel economy improved significantly. Before suggesting specific fixes, could you share: Current mileage, Any recent maintenance work, Whether you hear any unusual sounds during acceleration, If the CEL is constant or intermittent, What engine codes are showing These details would help determine if your issue is identical to what I encountered. A proper diagnosis is crucial since vacuum-related problems can sometimes present similar symptoms but have different root causes.

Last serviced at 170714 KM in December. After reading through both responses, I can confirm that my boost controller is indeed showing similar behavior. When hooked up to diagnostic equipment, it revealed unstable pressure readings. Another interesting detail I noticed is that the pressure drop seems more pronounced during cold starts. The pressure sensor readings are erratic, especially during the first few minutes of operation. The workshop initially suggested checking the N75 valve, but given the symptoms described, I think the vacuum switching valve deserves a closer look. Thanks for the detailed breakdown of costs and repair outcomes. It sounds like a promising direction to investigate.

Thanks for the additional details about the cold start behavior, that really strengthens the vacuum switching valve theory. On my RS3, I also noticed the pressure fluctuations were most severe during cold operation before the repair. The erratic pressure sensor readings you mentioned are a telltale sign. When my boost solenoid was tested, it showed similar inconsistent pressure patterns, particularly at startup. The intake manifold pressure would swing wildly before stabilizing. While the N75 valve is worth checking, my money is on the vacuum switching valve based on the identical symptoms. The engine performance issues during cold starts followed by gradual stabilization matches my experience exactly. I would suggest having both components tested since a failing pressure sensor can sometimes mask other issues. In my case, the total diagnostic time was about 2 hours before they pinpointed the exact source of the vacuum leak. Just to confirm, are you also getting fault code P0299? That was the main code my system threw before the repair when the boost pressure control was malfunctioning.

As the original poster, I can see the symptoms match up perfectly with what you experienced. You're right about the P0299 code, it's exactly what I'm getting, along with occasional P0234 during cold starts. The bypass valve definitely seems to be the prime suspect, especially given how the boost pressure fluctuates during initial startup. I had the boost solenoid tested yesterday and the readings showed major pressure variations, matching what you described about the compressor surge symptoms. Based on all this information, I will schedule a proper inspection of the vacuum switching valve and associated components. Your repair experience gives me confidence that this is likely the root cause rather than just a faulty N75 valve or sensor issue. The cost seems reasonable too at around 140 Euro for the fix. Will report back once I get it sorted. The cold start pressure drop and erratic sensor behavior really seems to be the smoking gun here.