1. Normal candle.
Brown or grayish-yellowish color and slight wear of the electrodes. Exact match of the heat value of the spark plug to the engine and operating conditions.
Note: When replacing spark plugs with new ones, install spark plugs with the same characteristics from those recommended by the manufacturer.
2. Soot deposits.
Dry soot deposits indicate a rich air-fuel mixture or late ignition. Causes misfires, difficult starting and erratic engine operation. Check for clogged air filter, coolant temperature and intake air temperature sensors.
Note: Use a hotter spark plug.
3. Oil deposits.
Oily electrodes and spark plug insulator. Cause: oil getting into the combustion chamber. Oil gets into the combustion chamber through the valve guides or through the piston rings. Causes difficult starting, cylinder misfires and "twitching" of the running engine. Repair of the cylinder head and piston group of the engine is required. Replace the spark plugs.
4. Metal-containing plaque.
Brick-red iron oxide deposits on the insulator skirt from antiknock iron-containing additives (ferrocenes) to gasoline. They are deposited in an even, dense layer. When the engine is running under heavy load, under the influence of high temperature and pressure in the combustion chamber, the oxides are reduced to conductive paths of pure iron, shorting the central electrode to ground. This causes misfires and, as a consequence, a drop in engine power and increased fuel consumption. In addition, the catalytic converter of exhaust gases can be damaged, greatly overheating when gasoline that has not burned in the engine cylinders gets into it. The deposit is practically impossible to remove mechanically and does not burn out during prolonged movement at high speed. If this deposit appears on new spark plugs after a short mileage, change the refueling location.
Note: If you cannot immediately replace the spark plugs with new ones, try to remove this deposit by immersing the spark plugs with their insulator skirts in phosphoric acid or a rust converter (contains phosphoric acid) for ten minutes. After that, clean the deposit with a non-metallic brush (an old toothbrush will do) and wash the spark plugs first with water and then with gasoline.
5. Melted electrodes.
Early ignition. The insulator is white, but may be contaminated due to spark misfires and deposits from the combustion chamber. May cause engine damage. It is necessary to check the spark plug type, the cleanliness of the injector nozzles and fuel filter, the operation of the cooling and lubrication systems.
6. Ash deposits.
Light brown deposits that crust over the center and side electrodes. They come from oil or gasoline additives. Large amounts can insulate the spark plug electrodes, causing misfires and hesitation during acceleration. If excessive deposits form over a short period of time or at low mileage, replace the valve guide seals to prevent oil from entering the combustion chamber.
Note: If deposits are consistently formed over long mileage, the cause is the quality of gasoline - change the place of filling.
7. The spark plug insulator is cracked or chipped.
Cause: detonation, can damage the piston. Make sure the octane number of gasoline corresponds to the required one.
8. Mechanical damage to the electrodes and insulator of the spark plug.
Damage can be caused by foreign objects getting into the combustion chamber, and if a spark plug is too long, its electrodes can catch on the piston. This leads to the destruction of the spark plug, the cylinder being disconnected and can damage the piston. Remove the foreign object from the cylinder and/or replace the spark plug.