Audi S5 V8 Misfire: Common Causes and Solutions

I had a similar issue with my 2006 A5 4.2 V8. Having some experience working on these engines, I can tell you that cylinder misfire symptoms can be quite concerning. The check engine light was flashing on acceleration, and I noticed the same jerky behavior and rough idle you described. After scanning for codes, combustion misfire was confirmed in cylinder 4. Initially thought it might be ignition coils or spark plugs, but the real culprit turned out to be a defective fuel injector. The workshop diagnosed it using pressure testing of the fuel system and injection pulse measurements. They found the injector was not atomizing fuel properly, leading to incomplete combustion. Total repair cost was 515 Euro for parts and labor, including testing. Before jumping to conclusions, could you share: Have you scanned for specific error codes? Are you noticing the misfire more during cold starts? Does the fuel pump make any unusual noise when you turn the key to ON position? Which cylinder numbers are showing misfire codes? This information would help determine if your issue matches what I experienced. These engines can show similar symptoms for different underlying causes, so proper diagnosis is crucial.

Thanks for the insights. I did manage to get it scanned at last service (31318 KM) and it showed multiple misfires across cylinders 2 and 6. The throttle response definitely feels worse during cold starts. No unusual fuel pump sounds though. The spark plugs and ignition coils were actually replaced about 10000 KM ago, so I am starting to suspect it might be fuel injector related like in your case. The symptoms seem very similar.

Based on your description and cylinder pattern, this is sounding increasingly like a fuel delivery issue similar to what I experienced. Multiple cylinders showing combustion misfire codes, especially on cold starts, is a classic sign of fuel system problems in these V8s. Since your ignition coils and spark plugs are relatively new, the fuel injectors are the next logical suspect. On my car, the workshop found that injectors can deteriorate gradually, leading to poor atomization and incomplete combustion. This matches your symptoms perfectly, especially the cold start issues. The fact that cylinders 2 and 6 are affected could indicate either individual injector failures or a fuel pressure problem affecting that bank of cylinders. When my injector failed, the check engine light would flash more aggressively during acceleration, which is when fuel demand is highest. I would recommend: 1. Getting fuel pressure tested 2. Having injector spray patterns analyzed 3. Checking fuel trim values for those specific cylinders My repair involved replacing two injectors and cleaning the others, costing around 680 Euro. It completely resolved the rough idle and restored proper power delivery. The repair has held up well for 40000 KM now with no recurring misfires.

Thanks all for the helpful responses. Just wanted to close the loop on this one. Took it to the shop yesterday and as suspected, the fuel injectors were the culprit. They found injectors in cylinders 2 and 6 were not atomizing properly, causing the cylinder misfire issues. The throttle response was particularly bad because of incomplete combustion during cold starts. The shop did a full diagnosis including fuel pressure testing and injector spray pattern analysis. They ended up replacing both problematic injectors and cleaning the rest. Total cost came to 695 Euro which seems in line with what others paid for similar repairs. The engine now runs smooth with no misfires, even during cold starts. Power delivery is back to normal and the check engine light has stayed off. Best of all, the rough idle is completely gone. Will keep monitoring but seems like this was definitely the right fix. Good call on checking the fuel system even though the spark plugs and ignition coils were recently replaced. Saved me from replacing parts unnecessarily.