Q5 Diesel Boost Control Issue Draining Power & Fuel

I had a very similar issue with my 2012 Audi A6 3.0 TDI. As someone with moderate experience working with turbo diesels, these symptoms immediately pointed to boost control problems. The main culprit in my case was a defective wastegate valve in the turbocharger assembly. The workshop diagnosed it using pressure testing and MAP sensor readings. The whistling and hissing sounds you describe match exactly what I experienced, along with the increased fuel consumption and power loss. The repair involved replacing the wastegate valve and recalibrating the boost control system. Total cost was 860 Euro including parts and labor. The difference in performance after the fix was night and day, proper boost pressure restored, fuel economy back to normal, and no more strange noises. Before you head to a workshop, could you share: Current mileage on your Q5, Whether you notice the issues more during cold starts, If any other warning lights are active besides the check engine, Whether the boost issues started gradually or suddenly This information would help determine if you are dealing with the same wastegate problem or perhaps another component in the turbocharger system.

Thanks for the detailed response. My Q5 has 133493 KM and had its last major service about 6 months ago. The issues started quite suddenly after a long highway drive. No other warning lights besides the check engine light. The MAP sensor readings from my diagnostic tool show inconsistent boost pressure, especially under load. The whistling noise is definitely worse during cold starts. Your wastegate issue sounds very similar to what I am experiencing. The 860 Euro repair cost seems reasonable considering the symptoms. Did they need to remove the entire intercooler assembly to access the wastegate in your case? Also wondering if they found any additional wear on the throttle body or other components while doing the repair.

Thanks for the additional details. Based on your mileage and symptoms, this is looking even more like the wastegate issue I had. The sudden onset after highway driving is particularly telling, that is exactly how mine started too. In my A6, they did need to remove the intercooler to properly access the wastegate assembly. The turbo lag was getting worse by the day until they fixed it. During the repair, they found minor carbon buildup on the throttle body which they cleaned, but no serious additional wear. Since your MAP sensor readings are showing inconsistent boost pressure, that further confirms my suspicion. The cold start whistling is another classic symptom of wastegate problems in these engines. The turbocharger system on these models is generally reliable, but around 130000-150000 KM the wastegate actuator often starts showing wear. The good news is that replacing just the wastegate rather than the entire turbocharger assembly is usually sufficient. I would get it checked soon though, continuing to drive with improper boost control can lead to more expensive damage. The 860 Euro I paid was much better than potentially needing a full turbocharger replacement.

Thanks for all the info. This has been really helpful. The mileage similarity and identical symptoms make me confident it is indeed the wastegate. Will book it in next week for repair. Good to know about the intercooler removal being necessary. I will mention the carbon buildup possibility to the workshop as well since they will have easy access while doing the wastegate work. The pressure sensor readings from my diagnostic tool keep getting worse, so getting this fixed soon makes sense. Really appreciate the tip about potential damage from continued driving with faulty boost control. Will minimize driving until the repair is done to avoid risking the entire turbocharger. The 860 Euro cost is definitely preferable to a complete turbo replacement. Thanks again for sharing your experience, it has made the diagnosis much clearer.