VW Passat Diesel Engine Problems: Fix Poor Performance

I dealt with nearly identical symptoms on my 2004 VW Golf TDI. After experiencing rough running, stalling, and reduced power output with the check engine light illuminated, I took it to my regular workshop. The diagnostic scan revealed a faulty injection valve was causing inconsistent fuel delivery. The mechanic explained that contaminated fuel had damaged one of the injectors, leading to poor atomization and irregular combustion. This explained the rough running and power loss. The repair involved replacing the defective injector and performing a thorough fuel system cleaning. Total cost was 515€, which included parts, labor, and testing. After the repair, the engine ran smoothly again with proper power output and the check engine light stayed off. To help diagnose your case more accurately, could you share: Current mileage on your Passat, When you last changed the fuel filter, Whether you notice any black smoke from the exhaust, If the problem started suddenly or developed gradually, Any unusual noises when starting the engine These details would help determine if your symptoms match what I experienced with my injector issue. Regular fuel system maintenance and using quality diesel fuel can help prevent similar problems. Would definitely recommend having a professional diagnose the issue with proper diagnostic equipment rather than guessing at parts replacement.

Thanks for sharing your experience with the injection valve issue. After reading the diagnostic details and repair procedure, I think we might be dealing with different problems. In my case, the check engine light started flashing after my latest service 2 months ago. I also notice the engine runs especially rough when the air-fuel ratio seems off during cold starts. The mass airflow sensor readings might be incorrect since the engine smooths out briefly when I disconnect it. No black smoke, but the catalytic converter makes a rattling sound occasionally. I will get the diagnostic codes read to confirm, but do you think I should also check the intake manifold for possible leaks? My mechanic mentioned this could cause similar symptoms.

Thanks for those additional details about your Passat, they help paint a clearer picture. Looking back at my Golf experience, I initially also suspected the intake system before discovering the injection issue. The fact that your check engine light started after a service and the engine runs better with the MAF disconnected points toward a different root cause than what I had. The symptoms you describe strongly suggest an air-fuel mixture problem, but likely from the air intake side rather than fuel delivery. A leaking intake manifold could definitely cause these issues by allowing unmetered air into the system. The rich mixture condition would explain why disconnecting the MAF temporarily helps. When my Golf had the injector problem, disconnecting the MAF made no difference. Your throttle body might also need cleaning since carbon buildup can cause similar rough running symptoms, especially during cold starts. The rattling catalytic converter could be reacting to the improper air-fuel mixture. I recommend having your mechanic: 1. Check for intake manifold leaks using smoke testing 2. Test the oxygen sensor readings 3. Clean the throttle body and MAF sensor 4. Verify proper fuel pressure Would you know if they changed or cleaned any sensors during that service 2 months ago? Sometimes disturbing the MAF or oxygen sensors can cause them to read incorrectly.

Thanks for the detailed response and recommendations. After considering your points about the intake manifold and sensors, I decided to take the car to my mechanic today. The diagnostic scan showed a failing mass airflow sensor and a small vacuum leak at the intake manifold gasket. The oxygen sensor readings were also off, which explains the catalytic converter noise. You were right about the air intake being the main issue rather than the fuel injection system. They found the MAF sensor was contaminated during the last service, causing incorrect readings. The mechanic replaced the MAF sensor, resealed the intake manifold, and cleaned the throttle body. Total repair cost was 380€. The engine now runs perfectly smooth, even during cold starts. The check engine light is off and the rattling noise is gone. Really glad I didnt jump straight to replacing injectors or other expensive components. Thanks again for helping me narrow down the problem, this saved me time and money by focusing on the right area first.