S8 Rough Idle & Stalling Points to Faulty IAC Valve

Having experienced similar idle speed issues on my 2020 A7, I can relate to these symptoms. The stalling engine and rough idle indeed point toward the intake system, but not necessarily just the IAC valve. In my case, the primary culprit was severe carbon buildup in the intake manifold. The engine stalling happened more frequently when the engine was warm, and the idle control became increasingly erratic. The check engine light came on intermittently at first, then stayed constant. The proper fix required: Complete intake manifold removal, Professional cleaning of carbon deposits, Replacement of gaskets, IAC valve inspection and cleaning, Throttle body cleaning, New MAP sensor This was a significant job that took about 6-7 hours of labor. The carbon buildup was quite severe and could have led to more serious engine damage if left untreated. For an S8, being a higher performance vehicle, having this properly diagnosed and fixed is crucial to prevent further complications. Rather than just replacing parts, recommend having a thorough intake system inspection with proper diagnostic testing first. The car idle issues often stem from multiple connected components rather than a single part failure.

Thanks for sharing your A7 experience. That carbon buildup issue sounds very similar to what my S8 is going through. The throttle body cleaning and intake manifold work you mentioned would explain the rough idle and stalling. Did you remember roughly how much the complete job cost? And has everything been running smoothly since the repair? The idle speed on mine is getting worse by the day, and I want to make sure I address it before it causes more damage to the air valve system.

Thanks for asking about the follow-up. The repair turned out to be exactly what was needed, the IAC valve was indeed faulty and the intake was heavily contaminated. The total repair cost came to 425 Euro, which included a thorough cleaning of the entire intake system. Since having the work done, the idle speed has been absolutely stable with no more stalling engine issues. The air valve is functioning perfectly now, and the car idle stays consistent even when the engine is fully warmed up. The mechanic showed me the removed parts, and the carbon buildup was actually worse than what I initially experienced with my A7. Getting it fixed when I did likely prevented more expensive repairs down the line. The job took about 5 hours total, and they performed a complete system check afterward to ensure everything was working correctly. No check engine light since the repair, and the idle control is now working exactly as it should. Considering the complexity of the S8 engine, addressing these intake issues early was definitely the right call. A rough idle can quickly lead to more serious problems if left unchecked.

After getting the advice here, I finally took my S8 to a new mechanic who actually knew what they were doing. The diagnosis confirmed exactly what was discussed, severe carbon buildup and a failing air valve system. The entire job took about 6 hours. They cleaned the throttle body, removed all the carbon deposits from the intake, and replaced several gaskets. The air intake system needed a thorough cleaning too. What really impressed me was how they showed me all the dirty components they removed. Total cost came to 480 Euro, definitely worth every cent. The engine idle is now perfectly smooth, no more stalling engine issues, and the check engine light has stayed off. Such a relief to have it properly fixed instead of getting the runaround like at the previous shop. Would definitely recommend getting these symptoms checked out early before they cause bigger problems. My only regret is not doing this sooner.