Rough Running Audi A3: Rich Fuel Mixture Fix Needed

As someone with some hands-on experience maintaining cars, I dealt with nearly identical symptoms on my Audi A4 2.0 from 2006. The symptoms you describe match my experience perfectly, poor fuel economy, sluggish performance, and that dreaded check engine light. After diagnosis, it turned out one fuel injector was defective, causing a rich mixture condition. The faulty injector was disrupting the proper fuel-air ratio, which explained the rough running and increased fuel consumption. The oxygen sensor readings were all over the place due to this issue. The repair involved: Complete diagnosis of the intake manifold and fuel system, Replacement of one faulty fuel injector, Cleaning of the air filter and intake system, Full system reset and calibration Total cost came to 515 Euro including parts and labor. The car ran like new afterward, proper acceleration, normal fuel consumption, and no more check engine light. To help further with your specific case, could you share: Have you noticed any specific conditions when the symptoms get worse? Did you scan for error codes? Has there been any recent maintenance work done? Any unusual sounds or smoke from the exhaust?

Thanks for sharing your similar experience. My Audi is at 199627 KM now, and I had the oxygen sensor replaced during the last service about 6 months ago. After reading your reply, I checked the air filter and it was completely clogged. Cleaned it up but the issue persists. When accelerating, I notice thick black smoke from the exhaust and the fuel economy has dropped to nearly double the normal consumption. I ran a diagnostic scan showing P0172 code, system too rich. Going to have the fuel injectors tested next week since they have never been replaced or cleaned in all these years. The symptoms get much worse during cold starts and when the engine is under load. I will update once I get it fixed.

Thanks for the update on your Audi. The black smoke combined with P0172 code strongly points toward a severe rich mixture condition. On my A4, I noticed similar symptoms before the repair, including that thick black exhaust especially during cold starts. The clogged air filter was smart to check, but since the issue persists, the fuel injectors are likely the primary culprit. The air-fuel ratio being off explains all your symptoms perfectly. In my case, the mechanic found carbon deposits had built up significantly in the injectors, disrupting their spray pattern. A few things to consider before your appointment: Have them check the intake manifold for any vacuum leaks, Request testing of all spark plugs, as they might be fouled from running rich, The mass airflow sensor might need cleaning, Make sure they verify the previous oxygen sensor repair is functioning correctly The fact yours has never had injector service at 199K definitely supports they could be the root cause. My repair bill ended up being worth every cent for the improved performance and fuel savings. Keep us posted on what they find, would be good to know if your fix ends up being similar to mine.

Thanks for all the detailed feedback. After taking it to the shop, they confirmed multiple issues affecting the air-fuel ratio. The fuel injectors were indeed severely clogged, and the intake manifold had a small vacuum leak. The catalytic converter also showed damage from running too rich for an extended period. The total repair included: Cleaning and testing all fuel injectors, Replacing the damaged injector, Fixing the intake manifold leak, Installing a new catalytic converter, Replacing fouled spark plugs The bill came to 1250 Euro but the car runs perfectly now. No more black smoke, check engine light is off, and fuel consumption is back to normal. The acceleration is smooth again without any hesitation or rough running. Really glad I didn't wait longer to fix it, as the mechanic mentioned the rich mixture condition could have caused more expensive damage if left unchecked. Thanks again for helping point me in the right direction with the diagnosis.