1. Normal candle.
Brown or greyish-yellowish colour and slight wear of the electrodes. Precise matching of spark plug heat value to 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 retarded ignition timing. Causes misfires, difficult starting and erratic engine operation. Check if the air filter is clogged and if the coolant temperature and intake air temperature sensors are working properly.
Note: Use a "hotter" spark plug.
3. Oil deposits.
Oily spark plug electrodes and insulator. The reason is oil getting into the combustion chamber. Oil enters the combustion chamber through valve guides or through piston rings. Causes difficult starting, cylinder misfires and "jerking" of the running engine. The cylinder head and piston group of the engine need to be repaired. Replace the spark plugs.
4. Metal-containing plaque.
Brick-red iron oxide deposits on the insulator skirt from anti-knock iron-containing additives (ferrocene) 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, 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, as it overheats greatly when gasoline that has not burned in the engine cylinders gets into it. The plaque 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 filling location.
Note: If you cannot immediately replace the spark plugs with new ones, try to remove this deposit by soaking the spark plugs with their insulator skirts in phosphoric acid or a rust converter for ten minutes (contains orthophosphoric acid). After this, clean off the plaque with a non-metallic brush (you can use an old toothbrush) and wash the spark plugs first with water and then with gasoline.
5. Melted electrodes.
Early ignition. The insulator is white, but can be contaminated due to spark misfires and deposits from the combustion chamber getting on it. May cause engine damage. It is necessary to check the compliance of 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 encrusting the central and side electrodes. They are isolated from oil or gasoline additives. A large number of them can lead to insulation of the spark plug electrodes, causing misfires and interruptions in 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.
The reason is detonation, which can lead to damage to the piston. Make sure that the octane rating of the gasoline meets the required level.
8. Mechanical damage to the electrodes and insulator of the spark plug.
Damage can be caused by foreign objects entering the combustion chamber, and if a spark plug is too long, its electrodes may catch on the piston. This will cause the spark plug to fail, the cylinder to shut off and may damage the piston. Remove the foreign object from the cylinder and/or replace the spark plug.