Audi S7 Engine Stalling Due to Faulty Bypass Valve

As someone with moderate experience working on cars, I encountered nearly identical issues with my 2015 Audi S6. The symptoms you describe match what I experienced, engine overheating coupled with significant power loss during acceleration. After investigation, it turned out the root cause was actually a faulty thermostat that had caused damage to the bypass valve system. The overheating was stressing multiple components in the engine, creating a chain reaction of issues with the air intake and exhaust system. The repair was covered under warranty since the thermostat failure was documented as the primary cause, with the bypass valve issues being consequential damage. While the repairs would typically cost around 800 Euro, I paid nothing due to the warranty coverage. To help diagnose your specific case more accurately, could you provide: Current mileage, When the symptoms first appeared, Whether any modifications have been made to the engine, If any other warning lights are active, Details about recent maintenance history The similarities between our cars suggest this could be a known issue, but having these details would help confirm if we are dealing with the same root cause. Remember that continuing to drive with engine overheating can lead to severe engine damage and reduced fuel efficiency, so getting this diagnosed promptly is crucial.

Thanks for the detailed reply on this issue. My Audi has 32464 KM and had its last major service 3 months ago. I am noticing that coolant leak seems to be getting worse, leaving small puddles after parking. The power loss during acceleration has become more pronounced since I first posted. The air intake system seems to be making unusual sounds too, almost like a whistling noise. Will get it checked at a specialist this week since it sounds like the thermostat issue you mentioned could be causing multiple problems in my case too.

Thanks for providing those additional details about your S7. The coolant leak you mentioned, combined with the whistling noise from the air intake, strongly reinforces my initial suspicion about the thermostat being the root cause. When I experienced these issues, the coolant leak started small but quickly worsened, just like yours. The whistling noise from the air intake system is particularly telling, this was also present in my case and was directly related to the overheating causing pressure irregularities in the intake system. This symptom emerged just before my thermostat completely failed. With only 32464 KM, this is definitely premature component failure. Given your recent service, be sure to bring your maintenance records when you visit the specialist. In my case, showing regular maintenance history helped establish the failure wasn't due to neglect. Quick word of caution, the combination of active coolant leak and engine overheating you are experiencing could accelerate engine damage. If possible, minimize driving until your appointment, and keep a close eye on coolant levels if you must drive. Would be interested to hear what the specialist finds, as it could help others with similar power loss and engine stalling issues in these models.

Thanks for the input. Just got back from the specialist and wanted to close this thread with the outcome. They confirmed it was indeed a faulty thermostat that had damaged the bypass valve, exactly as suggested. The coolant leak was coming from a crack that developed due to the constant engine overheating. The total repair cost came to 920 Euro, covering the thermostat replacement, bypass valve repair, and fixing the coolant leak. They also performed a full inspection of the exhaust system and air intake to ensure no additional damage occurred. The fuel efficiency has returned to normal and the engine is running smoothly with no power loss during acceleration. No more whistling sounds or coolant leaks either. Definitely learned my lesson about acting quickly when engine overheating symptoms first appear.