General description
The spark plug consists of a central electrode, an insulator with a housing and a ground electrode(s). A spark jumps between the central electrode and the ground electrode, igniting the fuel-air mixture. Do not use spark plugs other than those recommended. The recommended type of spark plug is determined, among other things, by the heat rating.
The glow number characterizes the degree of thermal load of the spark plug. The lower the glow number of the spark plug, the higher the degree of its thermal load. At the same time, the spark plug removes heat better, thereby eliminating the possibility of glow ignition of the fuel mixture. A spark plug with a high thermal characteristic has the disadvantage that its self-cleaning temperature is higher. It is more prone to carbon formation, in particular when the engine often does not reach its operating temperature during operation, for example, driving in the city, short trips in winter.
The correct glow plug rating is determined by the vehicle manufacturer. There are spark plugs with one or more ground electrodes, with different thread lengths and thread diameters. When replacing a spark plug, only spark plugs that comply with the vehicle manufacturer's instructions must be installed.
The average service life of a spark plug varies. The electrode material also plays a significant role. Chrome-nickel alloy is characterized by very high thermal conductivity and high corrosion resistance. Silver has the best heat dissipation, and platinum electrodes have high corrosion resistance and resistance to burning. The service life of spark plugs is between 20,000 and 100,000 km, depending on the material of the electrodes and their number.
Depending on the engine design, there are two types of seals between the spark plug and the cylinder head.
The flat seal has an outer sealing ring that is not lost and is fixed to the spark plug body. With a conical seal, no additional sealing elements are required. In cases where installation space is limited, spark plugs with a flat seating surface and a small spanner size are used in most cases. Spark plugs with a conical seating surface have smaller dimensions due to their compact design.
Spark plugs should be changed every 60,000 km (see Chapter Routine maintenance).
The appearance of the spark plug can be used to draw conclusions about the operation of the spark plugs, the quality of the mixture and the condition of the engine (pistons, piston rings, etc.)
Coal deposits
Symptoms: The presence of soot indicates an over-rich fuel-air mixture or weak spark intensity. Causes misfires, difficult starting and leads to unstable engine operation.
Recommendations: Check if the air cleaner is clogged, if the fuel level in the float chamber is too high, if the air damper is stuck and if the contacts are too worn. Try using spark plugs with a longer insulator, which increases resistance to contamination.
Oiling
Symptoms: Oiling of the spark plug is caused by worn out oil deflector caps. Oil gets into the combustion chamber through worn valve guides or piston rings. Causes misfires, difficult starting and leads to unstable engine operation.
Recommendations: Perform mechanical repairs and replace spark plugs.
Normal condition of the candle
Symptoms: Grey-brown color and slight wear of the electrodes. The glow number of the spark plugs corresponds to the type of engine and its general condition.
Recommendations: When replacing spark plugs, install spark plugs of the same type.
Ash formation
Symptoms: Soft, brownish deposits on one or both spark plug electrodes. These are caused by oil and/or fuel additives. Excessive buildup can insulate the electrodes and cause misfires and rough running under acceleration.
Recommendations: If deposits accumulate quickly, replace the oil seals, which will prevent oil from entering the combustion chambers. Try changing the fuel brand.
Overheat
(Article borrowed from the web portal: MercedesMan)
Symptoms: Porous, white insulator, erosion of electrodes and absence of any deposits. Leads to a reduction in spark plug life.
Recommendations: Check if it meets the requirements Specifications the glow plug rating, whether the ignition timing is set correctly, whether the fuel-air mixture is too lean, whether there are any vacuum leaks in the intake manifold and whether the valves are jammed. Also check the coolant level and whether the radiator is clogged.
Wear
Symptoms: Rounding of the electrodes with a small accumulation of deposits at the working end. Color is normal. Causes difficulty starting the engine in cold, wet weather and increased fuel consumption.
Recommendations: Replace the spark plugs with new ones of the same type.
Too early ignition
Symptoms: melted. The insulator is white, but may also be contaminated due to misfires or foreign particles entering the combustion chambers. May cause engine failure.
Recommendations: Check the glow number of the installed spark plugs, the ignition timing, and the quality of the mixture (isn't it too poor?), whether the cooling system is clogged and whether the lubrication system is functioning normally.
Detonation
Symptoms: Insulators may be chipped or cracked. Insulator damage may also result from improper spark plug gap adjustment. Piston damage may result.
Splashing
Symptoms: After a long period of misfire, deposits may become loose while the combustion chamber remains at operating temperature. At high speeds, deposits flake off the piston and stick to the hot insulator, causing misfires.
Recommendations: Replace the spark plugs or clean and reinstall the old ones.
Conductive gloss
Symptoms: The insulator has a yellowish color and a polished appearance. Indicates a sudden increase in temperature in the combustion chambers during sharp acceleration. Normal deposits melt, acquiring the appearance of a varnish coating. Leads to misfires at high speeds.
Recommendations: Change the spark plugs (cooler, while maintaining the same driving style).
Mechanical damage
Symptoms: Can be caused by foreign material entering the combustion chamber or by the piston hitting a spark plug that is too long. Leads to cylinder failure and piston damage.
Recommendations: Remove foreign particles from the engine and/or replace
Short circuit of electrodes
Symptoms: Combustion waste gets into the interelectrode space. Solid deposits accumulate, forming a bridge between the electrodes. Leads to ignition failure in the cylinder.
Recommendations: Remove deposits from the interelectrode space.
When screwing in the spark plugs, make sure that they are not overtightened, as this may damage the sealing washer. Do not exceed the specified Specifications tightening torque.