Megane engine knocking sound

Your description of a knocking sound coupled with reduced power, jerking, and smoke strongly suggests a combustion-related issue. I experienced a similar predicament with my old Opel Astra, albeit a different engine type. It turned out to be carbon deposits within the combustion chamber, disrupting the normal combustion process. Such deposits can indeed lead to pre-ignition or knocking, as you suspect. Have you checked the condition of your spark plugs? In my case, they were heavily fouled.

Thanks for the quick reply! Fouled spark plugs, that's a good point. I haven't checked them yet, but I will. When you say carbon deposits, how exactly did you diagnose that? Did you have to take apart the engine, or is there another way to check?

In my case, the diagnosis involved a combination of factors. The symptoms were highly indicative, and my mechanic performed a compression test, which revealed inconsistencies across the cylinders. While a borescope inspection can provide visual confirmation without complete disassembly, it is often difficult to perform at home. Given the symptoms and the stored error code, I would strongly advise consulting a qualified technician. Continuing to drive with knocking combustion may lead to consequential engine damage.

Thanks for the advice, I appreciate it! I took it to a local mechanic. He confirmed it was carbon deposits in the combustion chamber. He cleaned everything up, and the car runs like new now. Cost me about 425€.