S8 Engine Knocking and Smoke Points to Detonation Issue

I had a similar issue with my 2004 A8. As someone with moderate experience working on cars, I can share my troubleshooting journey. The symptoms you describe match what I experienced, jerky acceleration, smoke, and power loss. It turned out to be severe deposits in the combustion chamber, which was causing pre-ignition and detonation issues. The buildup on the cylinder heads was creating hot spots that triggered irregular combustion. My repair involved: Complete combustion chamber cleaning, Checking and adjusting valve timing, Replacing spark plugs, Fuel system cleaning The total cost was 425 Euro for parts and labor. Since the repair, the engine has run smoothly with no piston slap or irregular combustion issues. To better assist you, could you provide: Current mileage, Fuel type typically used, Specific error codes stored, Maintenance history, especially regarding spark plugs, Any unusual sounds during cold starts These details would help determine if your issue matches what I encountered and whether a similar repair approach might work for you.

Thanks for sharing your experience. My S8 has 60754 KM and just had its annual service last month. Based on your feedback, I checked the service records, the spark plugs were replaced about 40000 KM ago. The error codes point to cylinder misfire and knock sensor issues. The ignition timing seems off, particularly during hard acceleration. The symptoms match closely with what you described regarding pre-ignition and detonation. When cold, the engine also makes concerning noises that could indicate developing piston slap. I will schedule a combustion chamber inspection and cleaning based on your experience. The deposit buildup explanation makes sense given the age of the engine.

Thanks for the additional details about your S8. Looking at your mileage and the timing of the symptoms, your situation mirrors mine even more closely than I initially thought. The knock sensor codes combined with misfire issues strongly suggest combustion chamber deposits, just like in my case. The cold start noises you mention might not be piston slap, in my experience it was actually carbon deposits breaking loose and causing erratic ignition timing before the engine warmed up. A few key points from my repair that might help you: Premium fuel with higher octane rating helped reduce symptoms temporarily, The cylinder heads showed significant carbon buildup when inspected, Knock sensor replacement wasnt necessary, cleaning resolved the false readings Something I didnt mention earlier: after the combustion chamber cleaning, my mechanic recommended running a fuel system cleaner every 15000 KM to prevent deposit buildup. This preventive step has kept the issue from returning. For reference, the complete service including parts ran about 480 Euro in my case. The results were immediate, no more jerking, proper power delivery, and clean combustion with no smoke.

The symptoms in my S8 have gotten worse since my last post. The engine knocking is now more pronounced, especially during cold starts. I had the combustion chamber inspected today following your advice, and you were spot on, heavy carbon deposits were found. The mechanic confirmed the knock sensor was actually functioning correctly but responding to real detonation caused by these deposits. The total estimate for cleaning the combustion chamber, adjusting ignition timing, and performing a complete fuel system service came to 495 Euro. They also recommended switching to higher octane fuel for better engine tuning results after the service. The work is scheduled for next week. Thanks again for sharing your experience, it helped avoid potentially more expensive repairs by catching this early. The similarities between our cases, right down to the false assumption about piston slap, made your advice particularly valuable. Will update once the service is complete.