Contents: Normal condition of spark plugs ↡ The candles are sooty ↡ The candles are oily ↡ Overheating of spark plugs ↡
The spark plug thread diameter is 14 mm. Always use the manufacturer's recommended spark plug types.
Spark plugs are changed quite often, so it is not practical to name a specific type of spark plug. When purchasing new spark plugs, it is better to follow the data in the latest catalogs. The gap between the spark plug electrodes is 0.8 mm.
After 10,000 km of running, the spark plugs must be cleaned with a strong jet of sand and the gap between the electrodes must be adjusted. The gap is adjusted by tightening the side electrode. Impact on the central electrode is not allowed, as this can cause damage to the insulator.
Before unscrewing the spark plugs, check that there are no foreign objects in the recesses of the spark plug holes. If washers, bolts, stones or other objects fall into the spark plug hole after they are unscrewed, this may result in damage to the valves, valve seats or cylinder head after the engine is started.
By the appearance of the spark plugs, you can assess their condition and operation, as well as the carburetor adjustment, the adjustment of the fuel injection system, the composition of the fuel mixture and the condition of the engine (pistons, piston rings, etc.). Below is a description of the appearance of spark plugs and possible engine malfunctions:
Normal condition of spark plugs
The insulator color is from yellow-brown to brown, most often with a powdery coating. In addition to carbon deposits, the electrodes are covered with a powdery coating of brown-yellow or brown color. The inner surface of the spark plug body is light brown, yellowish or black-brown. The engine is in good condition. The spark plugs are selected correctly.
The candles are sooty
The insulator, electrodes and inner surface of the housing are covered with a thick, powdery, velvety coating of black-brown color. The reasons may be a rich mixture (lack of air), long operation of the starting valve, increased gap between the spark plug electrodes, spark plugs with a high glow number (remain cool during operation).
The candles are oily
The insulator, electrodes and body of the spark plug are covered with greasy, oily soot. Formation of oil carbon deposits. The cause is the penetration of oil into the combustion chamber, wear of the cylinder and piston.
(Publication taken from an online resource: mercedesman.ru)
Overheating of spark plugs
On the insulator there is a glazed and rough baked-on coating of dark brown or black-gray color with a crust on the end of the insulator. Wear of the electrodes, especially the central one. The surfaces are rough or corroded. The cause may be a lean mixture, damaged spark plugs, leaky valves, or spark plugs with a low heat rating were selected, which leads to their overheating.
When using leaded fuel, the insulator is normally grey. Clean the gap between the central electrode insulator and the housing with a strong jet of sand. When screwing in the spark plug, check the threads on the spark plug in the cylinder head for cleanliness.