You may have already solved this issue, if not check-out Walnut Shell Blasting or Media Blasting. This was one of the remedial fixes for Carbon Build Up on BMW 335d and 335i models, not mention other Direct Injected engines. I have had good success with BG products as a preventive measure for Carbon Build Up on Direct Fuel Injectors as well.Ahhh i have seen the round things. Yeah it seems the back 2 valves have the most oil/carbon. I'll have to see if i can replace them since everything is off anyways, or do you have to replace the entire valve cover?