Table of contents: Flawless appearance 🠇 Carbon deposits on spark plugs 🠇 Oiling the candle 🠇 Overheating of spark plugs 🠇
The manufacturer recommends types of spark plugs, but very often these recommendations change during production. It is therefore recommended to follow the latest recommendations when replacing spark plugs, that is, when purchasing new spark plugs, always indicate the engine type and year of manufacture. There are "cold" and "hot" spark plugs. It's best to consult a specialist before replacing them.
The interelectrode distance of the spark plugs is 0.7 - 0.8 mm and is measured in the manner shown in Figure 175.
Figure 175. The interelectrode distance is measured between arrows "A".
When checking spark plugs, the required value of the interelectrode distance is set. In no case should you bend the central electrode, as this may cause the porcelain insulator to crack. To maintain engine power, spark plugs are replaced every 20,000 km.
Before removing the spark plugs, check that there are no foreign objects in the recesses of the receiving holes. A washer, screw, stone, etc., falling into the hole when unscrewing the spark plug can damage the valves, valve seats or cylinder head the first time the engine is started.
Based on the appearance of the spark plug, one can draw conclusions about its suitability and normal operation, the composition of the mixture, the normal operation of the fuel injection system and the condition of the engine (pistons, piston rings, etc.). The general rules in this case are:
Flawless appearance
The base of the insulator is covered with a thin, yellow-gray to brown, often powdery coating. On the electrodes, in addition to the burnt surfaces, there is a yellow-gray to brown powdery coating. The inside of the body has a light brown or yellowish to blackish-brown coating. The engine is normal. The glow plug heat range is selected correctly.
Carbon deposits on spark plugs
The base of the insulator, the electrodes and the inside of the housing are covered with a thick, powdery, black-grey, resinous coating. The reason for this is a rich mixture, insufficient air supply, increased interelectrode distance, spark plugs have an increased heat rating and remain cold during operation. To eliminate this, install a spark plug with a different heat rating.
Oiling the candle
The base of the insulator, the electrodes and the inside of the housing are covered with a greasy, oil-slick coating. Formation of oil deposits. The cause may be oil penetration into the combustion chambers or wear of the cylinders and pistons.
Overheating of spark plugs
The base of the insulator is covered with a dark brown to grey-black, glassy or coarse, freshly baked coating, often with strong growths at the base of the insulator. The electrodes, especially the central electrode, are damaged. The surface is often rough, swollen, and corroded. The cause could be a lean mixture, poorly closing valves, or a spark plug with a low heat rating, which causes it to overheat. The deposits between the porcelain insulator and the spark plug body are removed by sandblasting. When screwing in a spark plug, be sure to thoroughly clean the threaded part first. Since the average spark plug lifespan is 20,000 km, cleaning every 10,000 km is sufficient. When installing the spark plug, do not overtighten it, as this will damage the sealing washer. The spark plug tightening torque value is from 18 to 28 Nm.