MTFCA Forum

Just got back from a tour and pulled the plugs to see how they fared. They were
covered with this yellow stuff which I have never seen before. Any ideas of
what is going on here would appreciated.

Craig Bothwell
bothwell.craig@gmail.com

The stuff you call as yellow is generally more brown. It is porcelain from the insulator getting burned off by a lean condition in the cylinder. Run a little richer or get a colder spark plug.

Thin, light tan to brown deposits are normal, especially with open road driving. Extremely lean fuel mixtures, as noted above, can cause lighter colored deposits, or even blistering of the insulator. Another thing that can affect the appearance of the plugs is dust. If the engine is ingesting road dust, especially when running under load, the combustion heat and pressure can cause the dust particles to fuse to the spark plugs and exhaust valves. This is typically a thicker deposit that looks like brown, orange, or tan unglazed pottery. Carbon or soot deposits from oil burning or an over-rich fuel mixture are black. Some gasoline dyes or other additives may affect the color of combustion chamber deposits.

An excessively lean mixture can be hard on exhaust valves and will lower power output and cause issues like lean backfire and an increased tendency to overheat. Ford did encourage operators to lean out the mixture, consistent with good performance, once the engine was warmed up, for best economy and reduced carbon deposition. That said, going to extremes can cause poor drive-ability and may shorten valve life. Running too rich is also to be avoided, since it wastes fuel, reduces performance, increases combustion chamber deposits, and can cause excess engine and transmission wear due to diluting the crankcase oil and washing the oil film off the upper cylinder walls. There is a narrow range in the mixture adjustment where the engine performs best, once warmed up, and Ford provided the mixture adjustment control to allow the driver to keep the mixture within that range.