Cold Fouling/Carbon Fouling

Cold fouling is sometimes referred to as carbon fouling. The deposits that cause cold fouling are basically carbon. See Figure. A dry, black deposit on one or two plugs in a set may be caused by sticking valves or defective spark plug cables. Cold (carbon) fouling of the entire set of spark plugs may be caused by a clogged air cleaner element or repeated short operating times (short trips).