Audi S4 Turbo Issues: Whistling & Power Loss Fixed

I had a very similar issue with my 2019 Audi S5 diesel last year. Being someone with moderate experience working on turbocharged engines, I initially thought it was related to the intercooler system. The symptoms were nearly identical, turbo lag increased significantly, boost pressure was inconsistent, and I noticed the same whistling noise during acceleration. After monitoring the boost control readings, the workshop diagnosed a defective wastegate valve, which was causing irregular boost pressure and the performance issues you described. The repair involved replacing the wastegate actuator and recalibrating the boost control system. Total cost was 860 Euro including parts and labor. Since the fix, the turbocharger performance returned to normal and fuel consumption improved significantly. To better assist with your specific case, could you share: Current mileage on your S4, Any recent maintenance or modifications, Whether the issues appeared suddenly or developed gradually, What boost pressure readings you are getting, Any additional error codes from the diagnostic scan This information would help determine if you are dealing with the same wastegate issue or potentially something else in the turbocharger system.

Last service was at 71000 KM and the pressure sensor readings have remained stable until this issue started. The boost pressure drops significantly under load, showing around 0.8 bar when it should maintain 1.8-2.0 bar. The whistling gets worse when the throttle body is fully open. After checking the intercooler for leaks and finding none, Im now suspecting either the wastegate or a failing turbo. Planning to have it properly diagnosed at a shop next week.

Based on those boost pressure readings and symptoms, I am even more convinced your issue mirrors what I experienced with my S5. The significant pressure drop under load, especially with an intact intercooler system, strongly suggests a wastegate malfunction rather than a failing turbocharger. The throttle body correlation with increased whistling matches my experience exactly. When the wastegate started failing on my car, the noise became more pronounced at full throttle because the turbocharger could not properly regulate boost pressure. Since your MAP sensor readings were stable before this started, and the intercooler shows no leaks, this further supports a wastegate issue. In my case, the faulty wastegate actuator caused similar pressure drops from 2.0 bar down to around 0.9 bar under load. Getting it diagnosed is the right move. If it is the wastegate, the repair should be straightforward, though I recommend having them check the turbocharger shaft play while they are at it, just to rule out any bearing wear that could cause issues later.

Thanks for the detailed response. I will definitely mention these symptoms to the shop and ask them to inspect both the wastegate and turbo bearings. The pressure readings you shared from your experience are nearly identical to what Im seeing, which gives me hope its just the wastegate and not the entire turbocharger that needs replacement. The MAP sensor data showing stable readings until recently also matches what you described. After looking more closely at the intercooler and finding no obvious leaks or damage, your wastegate theory makes the most sense. Will update once I get the official diagnosis next week.