If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning optimally.Īlthough there are many different cases, if the insulation resistance between the center electrode and the shell is over 10 ohms, the engine can be started normally. Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and possible engine damage.Īn engine’s condition can be judged by the appearance of the spark plug’s firing end. 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. 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. In general, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition.
By examining the insulator firing nose color, an experienced engine tuner can determine a great deal about the engine’s overall operating condition. Being able to “read” a spark plug can be a valuable tuning aid.
