Poor A4 TDI Performance Points to Cylinder Issues

Dealt with similar symptoms in my A4 3.0 TDI from 2005. Initially thought it was just the spark plugs, but diagnostic tests revealed a failed turbocharger caused by low oil pressure. The root cause traced back to a poorly performed oil change where debris was left in the system. This created a chain reaction, contaminated oil damaged the turbocharger, which then affected cylinder compression and fuel efficiency. The damaged turbocharger was sending excess fuel into the combustion chamber, leading to poor performance and high consumption. The check engine light came on due to the system detecting irregular fuel-air mixture. This was indeed a serious repair job requiring: Complete turbocharger replacement, Oil system flush, New oil pump, Cleaning of the intake manifold The symptoms match what you describe, poor performance and high fuel consumption often point to issues with air-fuel delivery. Given your mileage, the piston rings could be a factor, but I would first check the turbocharger and intake system. These components are more commonly the culprit with those symptoms in TDI engines. Recommend finding a shop specializing in VAG diesel engines, specifically someone experienced with the common rail injection system.

Thanks for sharing your experience. That helps a lot. The symptoms sound exactly like what I am experiencing with my A4. The turbocharger issue makes sense given how the car is performing. The fuel consumption is really worrying me, almost double what it used to be. Did you remember roughly how much the entire repair cost? Also wondering if you had any other issues pop up after fixing the turbocharger and cleaning the fuel injectors? Hoping to avoid any surprises if I need similar work done. I was worried about the piston rings but your explanation about the turbocharger causing the high fuel use makes more sense. Going to look for a diesel specialist as suggested rather than risk another bad repair job.

After fixing everything, my A4 has been running perfectly. The cylinder pressure fluctuation that was causing the high fuel consumption completely disappeared once the turbocharger system was properly repaired. Like you, I was initially concerned about major engine components like piston rings, but the real culprit was the damaged turbocharger affecting the entire fuel delivery system. The head gasket was also inspected during the repair since it can be affected by pressure issues, but thankfully it was in good condition. The mechanics also did a thorough check of the fuel injectors and intake manifold while the system was opened up. The best part was that since my issues stemmed from their previous error with the oil change, the shop covered all repair costs. They acknowledged their mistake and made it right at 0 Euro total cost to me. This included the new turbocharger, system cleaning, and all associated labor. If you find a reputable diesel specialist, they should be able to diagnose the exact cause. Based on your symptoms, it sounds very similar to what I experienced with the turbocharger system.

Thanks everyone for the helpful replies. After reading about your similar experiences, I bit the bullet and took it to a diesel specialist across town. Turns out you were spot on, the turbocharger was shot and needed replacement. The mechanic found metal particles in the oil, confirming the diagnosis. The total repair came to 2300 Euro including: New turbocharger unit, Complete oil system flush, Fresh filters all around, Labor costs Been driving it for about a week now and what a difference. The engine knocking is gone, fuel consumption is back to normal, and it pulls strong again. No more check engine light either. The camshaft and other components checked out fine during inspection. Lesson learned about sticking with specialists for these kinds of issues. The higher upfront cost was worth it to get it fixed right the first time. Really glad I got that second opinion rather than going back to my previous mechanic.