Rough-Running Audi S8: Lean Condition Steals Power

I had nearly identical symptoms with my 2008 Audi S6. After some initial troubleshooting, the check engine light and rough running turned out to be caused by faulty fuel injectors. Being somewhat experienced with engine management issues, I first checked the mass air flow sensor and oxygen sensor readings with a diagnostic tool, but the root cause was definitely the injectors. Took it to a specialized workshop where they confirmed one fuel injector was completely defective, causing the lean condition and power loss. They recommended replacing all injectors as a set to ensure consistent fuel delivery. Total cost was 515 Euro including parts, labor and programming of the new injectors to the engine control unit. The repair completely resolved the jerky acceleration and restored full power. The check engine light has stayed off since then. The throttle response is now smooth and predictable again. What diagnostic codes is your S8 showing? Have you noticed if the symptoms get worse when the engine is cold or at specific RPM ranges? This information would help determine if you are dealing with the same injector issue.

Last service was at 65000 KM, about 2 months ago. The issue started shortly after I had a full service. Looking at the OBD scanner, I am getting P0171 and P0174 codes, indicating system too lean on both banks. I noticed the symptoms are more pronounced when the engine is cold and especially between 2000-3000 RPM. The spark plugs were replaced during the last service, so I doubt those are the culprit. Given that the mass air flow sensor was cleaned during service, I am wondering if the throttle body could be the source of my problems. The rough idle and jerky acceleration seem consistent with throttle body issues I have read about. Would a bad throttle body cause these specific lean codes and symptoms? The fuel injectors and spark plugs being new makes me think the issue lies elsewhere in the air intake system.

Thanks for providing those details about the P0171/P0174 codes and symptoms. Having gone through this with my S6, I initially suspected the throttle body too, especially given the timing after your service. However, while a dirty throttle body can cause rough idle, the dual bank lean codes combined with cold-start issues match exactly what I experienced with failing fuel injectors. Even new injectors can be faulty, I learned this the hard way after initially ruling them out in my diagnostic process. The air-fuel ratio being off on both banks, especially during cold running, strongly suggests a fuel delivery problem rather than an air intake issue. The mass air flow sensor being recently cleaned actually helps narrow this down, as it rules out one common cause of lean conditions. Before spending money on the throttle body, I would recommend having the fuel pressure tested and fuel injector spray patterns checked. In my case, what appeared as a throttle body problem turned out to be injectors not atomizing fuel properly, causing the lean condition and rough running. Have you noticed any fuel trim values on your scanner? On my S6, the long-term fuel trim values were significantly positive before replacing the injectors, indicating the ECU was trying to compensate for insufficient fuel delivery.

Based on your scanner information and the fact that these issues started right after service, I understand the inclination to focus on the throttle body and air intake system. However, those P0171 and P0174 codes combined with cold-start problems are raising red flags about fuel delivery. Just checked the fuel trim values again, both short and long term are showing +25% adjustment, which suggests the ECU is desperately trying to add fuel to compensate for a lean condition. This matches what mentioned about their S6 injector problems. I will get the fuel pressure and injector spray patterns tested before making assumptions about the throttle body. The similarity tos experience with faulty new injectors is too close to ignore, especially with identical symptoms in the 2000-3000 RPM range. The oxygen sensor readings are all over the place during acceleration, which makes sense if the injectors are not atomizing properly. An intake manifold or EGR valve issue would likely show different symptoms. Will update once I have the injectors properly diagnosed at a workshop. Thanks for helping narrow this down, saved me from potentially replacing parts that arent actually causing the problem.