1. Occasionally, plugs with rust-colored burn marks on the surface of the insulator around the firing end are seen. Among the additives included in gasoline (for example the value-improving agent octane) there may be components like manganese. If compounds of manganese and other substances generated by combustion accumulate on the insulator around the firing end, the surface may become a rusty color. When heated, this coating may reduce the insulating performance and cause misfires.
2. When fitting spark plugs, some people apply an anti-seizure agent like molybdenum to the screw thread. (Never use such an agent: it can lead to the application of excessive torque and overtightening.) This anti-seizure agent may adhere to the insulator and, when heated, will also degrade insulation resistance and may cause misfiring.
