The ability to “read” a spark plug can be a valuable tuning aid. By closely examining the insulator firing tip color, you can determine a lot about an engine’s overall operating condition.
Generally, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition. Dark coloring, such as heavy black wet or dry deposits, can indicate an overly rich condition, too cold a heat range spark plug, a possible vacuum leak, low compression, overly retarded timing or too large a plug gap.
If the deposits are wet, it can be an indication of a breached head gasket, poor oil control from ring or valvetrain problems or an extremely rich condition—depending on the nature of the liquid present at the firing tip.
Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and possible engine damage.