S8 Rich Fuel Mix and Sluggish Performance Issues

Had similar issues on an A8 4.2 from 2005. The symptoms matched exactly, high fuel consumption, jerking during acceleration, and rough idle. After diagnostic tests, it turned out one fuel injector was defective, causing an incorrect air-fuel ratio and rich mixture condition. The defective injector was spraying too much fuel continuously, which explained both the high fuel consumption and engine performance issues. The catalytic converter was also starting to get damaged from the constant rich mixture. This was a serious issue requiring immediate attention since bad injectors can cause: Engine misfires, Damaged spark plugs, Damaged catalytic converter, Significantly worse fuel economy, Potential engine damage if left unfixed The repair involved replacing the faulty injector and cleaning the fuel system. The check engine light disappeared after the fix and the engine returned to running smoothly. Would recommend getting it checked soon, as running with a rich mixture can cause expensive damage to other components if left unaddressed. The symptoms are very typical of injector issues, especially in these engines after 100000+ km.

Wow, thanks for sharing your detailed experience. My S8s symptoms really do match yours with the rough running and fuel consumption issues. Good call about checking the injectors and the potential catalytic converter damage. Makes sense that a rich mixture would cause all these problems. I should probably get the oxygen sensor checked too since that could cause similar symptoms if its not reading properly. Did your mechanic mention anything about the air filter or intake manifold when doing the work? Mind sharing what the total repair cost ended up being? Also curious if youve had any other fuel system problems since getting it fixed.

Thanks for asking about the follow-up. Happy to report that after replacing the faulty injector (515 Euro total), all the rich fuel mixture issues were completely resolved. The engine now runs perfectly smooth, and fuel economy has returned to normal levels. The mechanic actually tested the oxygen sensor and intake manifold during diagnosis, both were working fine. The root cause was definitely the defective injector that was flooding the cylinder with excess fuel. This was affecting the entire fuel system performance and causing those rough running symptoms. Since the repair, the fuel system has been working flawlessly, no more jerking during acceleration or rough idling. The catalytic converter thankfully hadnt sustained any permanent damage, as we caught the problem in time. One thing to note, while checking the injector, they did a complete fuel system cleaning which helped ensure optimal performance after the repair. No other fuel-related issues have cropped up in the months since getting it fixed. Looking at your symptoms, its really similar to what I experienced. Getting it checked soon would be wise to prevent any potential damage to the catalytic converter or other components.

Thanks everyone for the helpful responses. Finally bit the bullet and took it to a mechanic last week. Turns out I also had a bad fuel injector plus two worn spark plugs causing the rich mixture issue. Total repair came to 680 Euro including parts and labor. The difference in how the car runs now is amazing. Fuel economy is back to normal, no more jerking when accelerating, and the engine idles perfectly smooth. The check engine light is gone too. Definitely worth getting it fixed before it caused more expensive problems. Really glad I asked here first, helped me go to the mechanic with a better understanding of the likely issue. The diagnostic process was much smoother since I could explain the symptoms properly and knew what to expect. Thanks again for steering me in the right direction!