I have a hard time understanding why carbon build ups are a problem on modern direct injection gasoline engines but not in diesels. I understand that there's no fuel constantly washing the valves but diesels are the same and this problem doesn't seem to appear or only marginally. I never heard of it and i live in a place where most cars are diesels since at least 25 years and oil used to be pretty crappy. Is there a correlation between the volume of air entering the engine (presence/lack of throttle body) and the oil deposits building up on valves ? Enlighten me.