Seat Formentor engine issues

Sounds like pre-ignition issues. I had similar symptoms in my old Opel Astra, loss of power, jerking. Have you checked your knock sensor? Sometimes a faulty sensor can cause the ignition timing to go haywire. In my case, it was a combination of a bad sensor and some carbon buildup.

In my Astra, the carbon definitely contributed. The bad knock sensor made the engine timing go nuts, which led to incomplete combustion and more carbon buildup. It became a vicious cycle. Cleaning the cylinder heads can help, but I'd suggest getting a proper diagnostic first. If your mechanic suspects deposits, they should be able to confirm that pretty easily. Best to get it sorted at the workshop.

Interesting. So, with your Astra, did the carbon build-up actually cause the pre-ignition, or was it just a contributing factor? I am wondering because the error memory showed a combustion issue, and the mechanic mentioned deposits in the combustion chamber being the most probable cause. Is cleaning the cylinder heads a potential solution?

I took the Formentor to the workshop. They confirmed carbon deposits in the combustion chamber were causing pre-ignition. They cleaned it, and the car runs perfectly again. Cost 425€, but it was worth it to get rid of that awful knocking and smoke. Thanks again for your help.