SQ8 TDI Power Loss and High Fuel Use Needs Diagnosis

I had nearly identical symptoms with a 2017 SQ7 TDI. Based on my experience working on diesel engines, the issue was related to a clogged fuel injector that caused uneven cylinder pressure. Initially, the engine knocking made me think it was something more serious with the piston rings. The root cause in my case was actually traced back to contaminated fuel that had damaged one fuel injector. Since this was a direct result of a previous fuel system repair done incorrectly at the dealership, they covered the repair cost completely. They cleaned the entire fuel system and replaced the faulty injector. After the fix, fuel consumption returned to normal and power was fully restored. The engine knocking disappeared completely. However, to better assist with your specific case, could you share: Current mileage on your engine, When was the last fuel system service done, Any recent repairs or maintenance, Type of fuel typically used, Any unusual sounds from the intake manifold area These details would help determine if your issue matches what I experienced or points to something different. Without having this information, its hard to say if your cylinder pressure issue stems from the same root cause or requires a different approach.

Thanks for sharing your experience. My mileage is currently at 46586 KM and last service was done only 2 months ago. The knocking sound you mentioned matches what I am experiencing, especially during cold starts. I have been using premium diesel exclusively from major stations. No recent repairs besides regular maintenance. Your fuel injector theory makes sense since the symptoms appeared shortly after that last service interval. The sudden power loss and increased fuel consumption align perfectly with a fuel delivery issue rather than internal engine damage to the piston rings or head gasket. The turbocharger seems to be working fine with no unusual sounds or boost issues, which further points to the fuel system. Will have the workshop check the injectors first before looking into more serious mechanical problems.

Those cold start symptoms you described definitely strengthen my initial suspicion about the fuel injectors. In my SQ7 case, the knocking was most noticeable during cold starts because thats when fuel atomization is most critical. Poor atomization from a faulty injector creates uneven cylinder pressure and that distinct knocking sound. Given your car has similar direct injection technology as mine, I would focus on the fuel system first. The timing of symptoms after your service is particularly telling. Sometimes tiny debris can get into the system during routine maintenance if proper cleaning procedures arent followed. When my fuel injector was replaced, they found microscopic metal particles had scored the injector tip. This created an uneven spray pattern affecting combustion and causing those pressure fluctuations. The camshaft timing was perfect and compression tests showed no internal engine damage. Before replacing parts, ask them to perform a fuel injector spray pattern test. In my case, this diagnostic step clearly showed the problematic injector and saved time checking other components unnecessarily. The workshop should also verify that the crankshaft sensor is reading correctly, as this can sometimes create similar symptoms, though in your case the correlation with the recent service strongly suggests a fuel system issue.

I really appreciate your detailed explanation about the fuel injectors and cold start behavior. The fuel injector spray pattern test you mentioned seems like the logical first diagnostic step, especially since no turbocharger issues are present. Your case history is particularly relevant since the symptoms started right after service, just like mine. The knocking sound during cold starts and the OBD readings showing cylinder pressure fluctuation match exactly what you experienced. Having this information helps rule out more severe issues with the head gasket. Going to schedule an appointment and specifically request they check the fuel injector spray patterns first. Will also have them inspect for any debris that might have entered during the last service. Makes more sense to start with the fuel system before exploring potential internal engine problems with the piston rings. Based on your experience, I feel more confident now that this is likely a fuel delivery issue rather than something more serious. Thanks for helping narrow down the problem and providing such specific details about your repair process.