If the insulation resistance drops to 0 ohms, the firing end is fouled by either wet or dry carbon. Overheating When a spark plug overheats, deposits that have accumulated on the insulator tip melt and give the insulator tip a glazed or glossy appearance. The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine's operating duration. Lead Fouling Lead fouling usually appears as yellowish brown deposits on the insulator nose. This can not be detected by a resistance tester at room temperature.
Lead compounds combine at different temperatures.
Breakage Breakage is usually caused by thermal expansion and thermal shock due to sudden heating or cooling. Abnormal Erosion Abnormal electrode erosion is caused by the effects of corrosion, oxidation and reaction with lead - all resulting in abnormal gap growth. Melting Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and uneven. The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface.