Audi S4 Turbo Failure Signs: Whistling and Power Loss

I had a very similar issue with my 2017 Audi S5 diesel last year. Being moderately experienced with turbo system issues, the symptoms you describe sound familiar. The whistling and hissing noises combined with power loss pointed to problems with the actuator valve. After diagnosis, it turned out the electrical bypass valve was defective, causing irregular boost pressure. The pressure sensor readings were all over the place, triggering the CEL. Had it fixed at an independent workshop specializing in turbo repairs. Total cost was 250 Euro including parts and labor. The repair took about 4 hours to complete. Before giving more specific advice, could you share: Current mileage on your S4, Whether the CEL is constant or intermittent, Any diagnostic codes you might have pulled, If the symptoms get worse during cold starts, Whether the boost pressure drops completely or fluctuates These details would help determine if your issue matches what I experienced with the actuator fault in my S5. The turbo system components are quite similar between these models, so the fix might be comparable.

Thanks for sharing your experience. My mileage is 109550 KM and I had the last service 3 months ago. The CEL stays on constantly and the boost pressure fluctuates rather than dropping completely. The symptoms are definitely worse during cold starts, with more pronounced whistling from the turbocharger area. I checked the codes with a basic scanner and got P0299 which apparently relates to turbo underboost conditions. Based on your experience with a similar turbo actuator issue, it sounds like I might be dealing with the same problem. The repair cost you mentioned seems reasonable if it turns out to be the bypass valve. Did your fuel economy return to normal after the fix?

Thanks for providing those details. Your symptoms align exactly with what I experienced, especially the P0299 code and cold start behavior. After dealing with my S5s turbo actuator fault, I can definitely say the fluctuating boost pressure and constant CEL match up. My fuel economy did return to normal within about a week after the repair. The turbocharger started delivering consistent boost again once the new bypass valve was installed and properly calibrated. Quick tip from my experience: While the bypass valve was being replaced, I had them inspect the whole boost control system, including vacuum lines and connections. Found a few minor vacuum leaks that were contributing to the whistling noise. Getting those fixed at the same time helped ensure the turbo system worked perfectly afterward. Before heading to the shop, recommend logging the boost readings if possible. This helped my mechanic pinpoint the exact issue with the actuator faster, which kept labor costs down. Based on your mileage and symptoms, it really does sound like the same bypass valve failure I dealt with. The 4-hour repair time I mentioned included testing afterward to verify proper boost control and turbo response. Just make sure whoever does the work recalibrates the new actuator properly.

Thanks for the thorough advice. Your details really helped me understand what I might be dealing with. The turbo actuator failure scenario fits all my symptoms perfectly, especially since I have the same P0299 code and cold start issues you mentioned. I will definitely have them check the entire turbo system including vacuum lines when they do the repair. Good point about logging the boost readings beforehand, I will get a proper diagnostic scan done first to help speed up the repair process. The 250 Euro cost and 4-hour timeframe seems quite reasonable for fixing the bypass valve and recalibrating the actuator. It is reassuring to hear your fuel economy went back to normal after the repair, as my current fuel consumption is really frustrating. I will book it in next week with a workshop that specializes in turbocharger repairs to make sure they can properly calibrate everything. Will update once I get it sorted to confirm if it was indeed the same turbo actuator issue you experienced.