An important point: inspection of spark plugs should be carried out after a long engine run, best after a long trip on the motorway. Very often, some motorists unscrew the candles to determine the cause of the unstable operation of the engine immediately after a cold start at a negative ambient temperature, and, having found black soot, make the wrong conclusion. Although, in fact, the cause of such carbon deposits is the forced enrichment of the mixture during engine operation in cold start mode, and the cause of unstable operation is the poor condition of high-voltage wires.
Therefore, as mentioned above, if deviations from the norm are detected in the operation of the engine, it is necessary to drive at least 250-300 km on initially clean candles, and only after that make diagnostics.
Photo #1 shows a spark plug removed from a normally running engine. The skirt of the central electrode is light brown in color, carbon deposits and deposits are minimal, and there are no traces of oil. Such an engine provides optimal fuel and engine oil consumption.
The spark plug shown in photo No. 2 is turned out of an engine with increased fuel consumption. The central electrode of such a candle is covered with a velvety black soot. This could be due to a rich air/fuel mixture (incorrect carburetor adjustment or malfunction of the electronic injection system), clogged air filter.
Photo No. 3 shows a spark plug from an engine whose air-fuel mixture, unlike the previous case, is too lean. The color of the electrode of such a spark plug is from light gray to white. When running on a lean mixture, the effective power of the engine drops. When using such a mixture, it does not go up to flame for a long time, and the combustion process occurs with disturbances, accompanied by uneven operation of the engine.
The skirt of the spark plug electrode shown in photo #4 has a distinctive brick red color. This coloration is caused by the operation of the engine on fuel with an excessive amount of additives containing metal salts. Prolonged use of such fuel leads to the formation of a conductive coating on the surface of the insulator. The formation of a spark will not occur between the electrodes of the candle, but at the place of the smallest gap between the outer electrode and the insulator. This will lead to misfiring and engine instability.
The candle, shown in photo #5, has pronounced traces of oil, especially on the threaded part. An engine with such spark plugs after a long period of parking tends to «triple», at this time a characteristic white-blue smoke comes out of the exhaust pipe. Then, as the engine warms up, the operation of the engine stabilizes. The cause of the malfunction is the unsatisfactory condition of the oil seals, which leads to excessive oil consumption. The process of replacing oil seals is described in the chapter «Mechanical part of the engine».
The spark plug shown in photo #6 is turned out of the idle cylinder. The central electrode of such a candle, as well as its skirt, are covered with a dense layer of oil mixed with drops of unburned fuel and small particles from the destruction that occurred in this cylinder. The reason for such a malfunction is the destruction of one of the valves or the breakage of the partitions between the piston rings with the ingress of metal particles between the valve and its seat, Symptoms of such a malfunction: engine «troit» without ceasing, a significant loss of power is noticeable, fuel consumption increases many times over. When such symptoms appear, it is impossible to delay the troubleshooting. Spark plugs should be inspected as soon as possible. To troubleshoot in the described case, an overhaul of the engine is necessary.
Photo #7 shows a spark plug with a completely destroyed center electrode and its ceramic skirt. The cause of such a malfunction could be prolonged operation of the engine with detonation, the use of fuel with a low octane rating, very early ignition, or simply a defective spark plug. The symptoms of engine operation are similar to the previous case. The car owner will be lucky if the particles of the central electrode can slip into the exhaust system without getting stuck under the exhaust valve, otherwise repair of the cylinder head cannot be avoided.
The spark plug shown in photo #8 has an electrode covered in ash deposits. In this case, the color of the deposits does not play a decisive role. The reason for this build-up is the combustion of oil due to wear or sticking of oil scraper piston rings. The engine has increased oil consumption, blue smoke is coming out of the exhaust pipe, the procedure for replacing piston rings is described in the chapter «Mechanical part of the engine».
The condition of the spark plugs is also recommended to be checked during scheduled vehicle maintenance. In this case, it is necessary to measure the gap between the electrodes of the spark plug and remove carbon deposits with a metal brush. Removing carbon deposits with a sandblasting machine can lead to microcracks, which later develop into more serious defects, which will eventually lead to the case described in photo # 7. In addition, it is recommended to swap spark plugs, since the temperature conditions of operation of different engine cylinders may not be the same (for example, the middle cylinders of engines with central fuel injection operate at higher temperatures than the outermost ones).