Audi Q7 Diesel Power Loss & High Fuel Consumption Fix

I had a very similar issue with my 2019 Audi Q5 diesel last year. As someone with some hands-on experience maintaining diesel engines, the symptoms you describe sound familiar. In my case, the fuel consumption increase and power loss were caused by a faulty crankshaft position sensor, which was actually a consequence of a previous timing belt replacement that wasnt done correctly. The workshop fixed it under warranty since it was their mistake during the earlier repair. Before jumping to conclusions about the fuel injectors or more serious issues, it would be helpful to know: 1. How many miles/kilometers are on your engine? 2. When was the last timing belt service performed? 3. Have you noticed any black smoke from the exhaust? 4. Are there any stored error codes you can share? 5. Has the car had any recent maintenance work done? The cold start issues and knock could indicate problems with the piston rings or head gasket, but in many cases, its something less severe. A proper diagnostic scan would help pinpoint whether its related to the fuel delivery system or engine timing components. The fact that your workshop mentioned cylinder pressure problems makes me think they might be heading in the right direction, but more information would help provide better guidance.

Last service was done at 140000KM and we replaced the spark plugs and timing belt. The turbocharger was also inspected and in good condition. The workshop pulled the error codes P0251 and P0234 which seem to point towards the fuel injection system. I also noticed during cold starts the engine runs rough for about 30 seconds before settling down. No black smoke from exhaust but fuel consumption has increased by about 30%.

Based on those error codes and symptoms, your issue is likely related to the fuel injection pump and boost pressure control. Since we have similar Audi models, I can share that my initial crankshaft sensor diagnosis was incorrect, it turned out to be a failing high-pressure fuel pump. The P0251 code specifically relates to the injection pump control circuit, while P0234 indicates overboost conditions. The rough cold starts you mentioned are a classic symptom of fuel delivery problems, which explains both the increased consumption and power loss. The timing belt replacement you mentioned is good, but the fuel system issues might have developed independently. In my case, the repair involved: 1. Replacing the high-pressure fuel pump 2. Cleaning the fuel injectors 3. Replacing several camshaft seals that were leaking The total repair cost was around 1200 Euro, but prices vary depending on parts availability. The engine knocking during cold starts completely disappeared after the repair, and fuel consumption returned to normal. Would suggest getting the fuel pressure tested, its a relatively simple diagnostic that can confirm if the high-pressure pump is the culprit before committing to more extensive repairs.

Thanks for the detailed explanation. I will schedule a fuel pressure test this week. Your experience with the high-pressure pump issues matches exactly what I'm experiencing, especially the cold start symptoms and power loss. The intake manifold and surrounding components were checked during my last service and were fine, so that helps narrow down the problem to the fuel system. Since my car is just over 140000KM, it makes sense that the high-pressure pump might be reaching the end of its service life. I will update the workshop about the potential fuel pump issue and get a quote for the complete repair job including injector cleaning. The 1200 Euro cost seems reasonable considering the complexity of the work. Glad to hear you got your issue resolved without needing a full engine overhaul. Will proceed with the fuel pressure testing and update once I have more information.