Spark plug with high glow value («hot» candle) gets very hot, which causes pre-ignition. Spark plug with low glow value («cold» candle) may not reach the required self-cleaning temperature, as a result of which the thermal cone of the insulator will be contaminated.
The gap between the electrodes and the length of the threaded part. In addition to the required glow value for the respective engine, the spark plugs must also have a certain gap between the two electrodes.
When replacing spark plugs, observe the following precautions:
- when removing the tips of the candles, pull not by the cable, but by the tip itself;
- when turning out the candles, fold them in the order of the cylinders. By the appearance of the candles, you can determine the failed cylinder;
- cold candles often sit very firmly. In this case, turn out the candles on a hot engine;
- screw in new candles on a cold engine;
- Tighten the spark plugs to 15 Nm. In the absence of a torque wrench, loosely screw the candles with a candle wrench until the sealing ring fits. If installing new spark plugs, tighten them a quarter turn with a spark plug wrench. If old spark plugs are installed, tighten them 15°.
By the appearance of the candles, you can judge the condition of the engine.
normal candle. The insulator is coated with a slight grey-yellow (to brown) raid. On the electrodes gray-yellow (to brown) deposits. The inside of the body has a light brown or yellowish (to black-brown) raid.
smoked candle. The thermal cone of the insulator, the electrodes and the inner part of the case are covered in most cases with a thick black-gray velvety coating.
Causes:
- very rich working mix;
- lack of air;
- too large a gap between the electrodes;
- very high heat number (candle too «cold»).
The candle is oiled. The thermal cone of the insulator, the electrodes and the body of the candle are covered with greasy soot with an oily sheen. The formation of oil soot.
The reason is the ingress of engine oil into the combustion chamber (wear of the cylinder-piston group, failure of oil scraper caps).
Candle overheating. The thermal cone of the insulator is coated with dark brown (to grey-black) glaze. In most cases, deposits are observed at the end of the thermal cone of the insulator. Causes:
- lean mixture;
- loosely wrapped candle;
- poorly closing valves;
- spark plug with too low glow value (candle too «hot»).
When using fuel with a lead additive, the thermal cone of the insulator will normally have a gray burn.
Deposits between the porcelain insulator of the center electrode and the spark plug body are cleaned with a jet of sand from a spark plug tester. When screwing the candles, clean the threads.
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