A5 Diesel: Sluggish Engine Points to Injector Issues

I had a very similar issue with my 2020 A4 diesel last year. Having some experience working on engines, I initially thought it was just a minor fuel system problem, but it turned out to be more complex. The engine knocking sound and performance issues you describe match exactly what I experienced. After diagnosis, it was revealed that a previous repair involving the timing chain had been done incorrectly, which led to cylinder pressure problems and affected the head gasket integrity. Since it was a consequential error from the previous repair, the workshop covered all repair costs. They had to adjust the timing, replace several components, and verify the piston rings were not damaged in the process. To help diagnose your specific case better, could you provide: Current mileage, Service history details, When was the last fuel system service performed, Any recent repairs or maintenance work, Does the knocking sound change with engine temperature The symptoms you describe could point to various issues, from minor sensor problems to more serious mechanical concerns. Best to have it properly diagnosed before the problem escalates and potentially causes more damage to engine components.

Thanks for the input regarding timing chain issues. My mileage is at 136369 KM and the spark plug replacement was done at the last service 3 months ago. The knocking sound definitely gets louder when the engine is cold and seems to improve slightly after warming up. The turbocharger appears to be functioning normally, but I notice the engine loses power particularly between 2000-3000 RPM. I will get it checked soon since it could be related to the camshaft timing or injector issues you mentioned. Better to address it early before it leads to more serious engine damage. Looking at the workshop schedule for next week. Will update once I have more concrete information about the root cause.

Thanks for providing those details. After dealing with similar symptoms on my A4, I can say the cold start knocking behavior combined with power loss in that specific RPM range strongly points toward fuel delivery issues. The timing between injector pulses and piston movement might be off, especially since this occurs in the 2000-3000 RPM range. When my fuel injectors started failing, I experienced nearly identical symptoms before getting them replaced. The crankshaft position sensor readings were also erratic, which contributed to the timing issues. Since your spark plug replacement was recent, we can likely rule that out. However, the fuel injector system should be inspected, particularly given the cold start symptoms and power loss you described. In my case, carbon buildup was causing irregular fuel spray patterns, leading to uneven cylinder pressure. Given the mileage, it would be worth having them check the timing belt condition while diagnosing the injection system. These components work together, and a worn timing belt can cause similar symptoms by throwing off the precise timing needed between the crankshaft and camshaft. Let us know what the workshop finds. Based on my experience, addressing fuel system issues early prevented more extensive engine damage.

Thanks for all the detailed feedback. After reading about your similar experience, I called the workshop and managed to get an appointment for next Tuesday. The symptoms you described with your injector system issues match my situation perfectly, especially regarding the cold start behavior. The service advisor mentioned they will first check the fuel delivery system and timing belt condition since the engine knocking gets worse during cold starts. They will also inspect the crankshaft sensor readings given the power loss issues I am experiencing. This seems like a sensible approach, particularly since my last timing belt inspection was over 60000 KM ago. I do not want to risk more serious engine damage by continuing to drive with these symptoms. While I hope it is just a minor injector issue, I prefer getting it properly diagnosed now. Will share the workshop findings once they complete their inspection. Thanks again for helping me narrow down the potential causes.