S8 Engine Stalling: Idle Issues & Fuel System Leaks

As an A6 3.0T owner from 2013, I experienced nearly identical symptoms, stalling at idle with check engine light. The issue was indeed the fuel tank ventilation system. The purge valve in the evaporative emissions system had failed, preventing proper fuel vapor management. My system failure triggered fault codes relating to incorrect fuel tank pressure. The mechanic confirmed the diagnosis using smoke-based leak detection testing, which revealed a defective activated charcoal filter and deteriorated seals. The repair involved replacing the complete purge valve assembly and charcoal canister. While not a major mechanical issue affecting engine integrity, it required prompt attention since: It caused poor idle stability, Increased emissions, Could potentially trigger fuel delivery problems, May fail emissions testing The repair process was straightforward but required specific diagnostic equipment to verify system integrity after parts replacement. The pressure sensor readings needed to be within spec to clear the check engine codes permanently. This is a common fault on VAG engines from that era, especially with higher mileage. Having it properly diagnosed with leak detection equipment is crucial to avoid replacing wrong parts.

Thanks for detailing your A6 experience. The symptoms match exactly what my S8 is doing. Glad to hear its likely just the purge valve and ventilation components rather than something more serious. After the repair, did your check engine light stay off for good? Also curious what the total cost ended up being for parts and labor. I want to budget appropriately and make sure I get everything fixed properly the first time. My local shop quoted me for leak detection testing but wondering if anything else came up during your repair that I should watch out for while they have it open?

After having the fuel tank ventilation system repaired on my A6, the emission control system has worked flawlessly. The check engine light cleared immediately and never returned. The repair focused on the defective activated charcoal filter which cost 275 Euro including labor and parts. The leak detection testing was essential for proper diagnosis and confirmed no other issues were present. Air flow and fuel efficiency returned to normal once the ventilation system was fixed. Make sure your mechanic verifies the system pressure readings are correct after the repair. No other components needed attention in my case, but its worth having them inspect the surrounding seals and connections while accessing the system. This preventive check can help avoid future system failure issues. Simple maintenance like this keeps these engines running smoothly, just ensure all emission control components are tested after repair to verify everything is working as designed.

Thanks everyone for the helpful responses. Just wanted to share how my repair went in case others run into this. Got my S8 fixed last week. The mechanic ran the leak detection test which found the purge valve had completely failed. They also discovered a cracked emission control hose that was causing additional pressure loss. The total repair came to 340 Euro including: New purge valve assembly, Pressure sensor calibration, Replacement hose, Labor for testing and installation Car runs perfectly now, no more stalling and the check engine light is finally off. Glad I had them do the full system check as that extra cracked hose could have caused problems later. Fuel efficiency has noticeably improved too. The repair took about 3 hours total. My mechanic showed me the old failed parts which helped understand what was actually wrong. Money well spent to get it done right with proper diagnostic testing first.