The spark plug ignites the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator.
The spark plug tip is a good indication of the combustion process.
Insulator nose has slight deposits. Color ranges from brown to greyish-white. Spark plug is functioning correctly under good engine conditions,
The centre and/or ground electrodes are worn. Deposits cover the insulator nose and ground electrode. This plug has operated under normal conditions for a considerable period of time and reached the end of it's life. Further usage will cause misfire, bad fuel economy and poor engine performance.
Wet, black, oily, carbon deposits form on the insulator nose and electrodes. Misfire results due to the reduction in insulation resistance between centre and ground electrodes.
Dry, soft, black carbon deposits form in large quantities on the insulator nose and electrodes. As carbon accumulates the insulation between the centre and the ground electrodes deteriorates, an elecitrical leakage path is formed by the carbon and misfire results.
The ground electrode is badly worn and appears to be chipped. Lead erosion is caused by the harsh lead additives in the fuel. At high temperatures these additives chemically react with the nickel alloy electrodes and separate the grain boundary of the nickel alloy.
The centre and/or ground electrodes badly worn relative to the length of operation. Misfire will eventually result due to the high ignition voltage needed for the large gap. Abnormal erosion is often caused by harsh lead and oil additives combined with severe operating conditions such as high speed driving.
Rapid Abnormal Erosion
The centre and/or ground electrodes are very badly worn. Their surfaces are fretted and rough. The electrodes have oxidised and sometimes turn green when the oxidation is heavy. Rapid abnormal erosion is produced by a more aggressive operative environment than 'Abnormal Erosion'.
Deposits (Ash Formation)
Heavy ash deposits build-up on the firing end and eventually cause misfire. In some circumstances these deposits can reach temperatures which might lead to pre-ignition. Ash formation is mainly produced from the burning of oil.
The insulator nose is cracked or split. Breakage not resulting from mechanical impact is caused by sudden heating or cooling (thermal shock). Severe detonation will often produce this type of breakage. Causes are similar to what is described in 'Overheated'
Overheated spark plugs have a white glazed or glossy insulator. Small black deposits accumulate on the insulator nose and the electrodes are prematurely worn.
Lead fouling usually appears as yellowish brown deposits on the insulator nose. The lead deposits have no adverse effect on starting, idling or low speed driving but do cause misfires during rapid acceleration or under heavy engine load. Misfire results when the lead deposits become electrically conductive at high temperatures creating a reduction in the insulating resistance between centre and ground electrodes. The lead additives used to enhance the fuel octane rating are the source of the lead deposits. Lead fouling will not occur with unleaded fuel.
The centre and/or ground electrode surface is uneven and cauliflower like in appearance. The insulator is blistered and possibly contains mettallic deposits. Melted firing end results from overheating. Refer to 'Overheating'.