An important point: the spark plugs must be inspected after the engine has been running for a long time, preferably after a long trip on the highway. Very often, some car enthusiasts unscrew the spark plugs to determine the cause of unstable engine operation immediately after a cold start at sub-zero ambient temperatures, and, having discovered black carbon deposits, draw the wrong conclusion. Although in fact, the reason for the formation of such carbon deposits is the forced enrichment of the mixture during engine operation in cold start mode, and the reason for unstable operation is the poor condition of the high-voltage wires.
Therefore, as mentioned above, if any deviations from the norm in engine operation are detected, it is necessary to drive at least 250-300 km with initially clean spark plugs, and only then carry out 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. This engine provides optimal fuel and engine oil consumption.
The spark plug shown in photo #2 was removed from an engine with increased fuel consumption. The central electrode of such a spark plug is covered with a velvety black deposit. The reasons for this may be 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 fuel-air mixture, unlike the previous case, is too lean. The color of the electrode of such a spark plug ranges from light gray to white; when operating on a lean mixture, the effective engine power drops. When using such a mixture, it does not ignite for a long time, and the combustion process occurs with disturbances, accompanied by uneven engine operation.
The spark plug electrode skirt shown in photo #4 has a characteristic brick red color. This coloration is caused by the engine running on fuel with an excessive amount of additives containing metal salts. Long-term use of such fuel leads to the formation of a conductive coating on the surface of the insulator. The spark will not form between the spark plug electrodes, but at the point of smallest gap between the outer electrode and the insulator. This will lead to misfires and unstable engine operation.
The spark plug shown in photo #5 has obvious traces of oil, especially on the threaded part. An engine with such spark plugs tends to misfire for a short time after being parked for a long time, with characteristic white-blue smoke coming from the exhaust. Then, as the engine warms up, its operation stabilizes. The cause of the malfunction is the unsatisfactory condition of the oil deflector caps, which leads to excessive oil consumption. The process of replacing the valve stem seals is described in the chapter "Mechanical part of the engine".
The spark plug shown in photo #6 has been removed from the non-firing cylinder. The central electrode of such a spark plug, as well as its skirt, are covered with a thick layer of oil mixed with drops of unburned fuel and small particles from the destruction that occurred in this cylinder. The cause of such a malfunction is the destruction of one of the valves or the breakage of the partitions between the piston rings with metal particles getting between the valve and its seat. Symptoms of this malfunction: the engine "triples" continuously, a significant loss of power is noticeable, fuel consumption increases significantly. If such symptoms appear, you should not delay in finding the fault. It is necessary to inspect the spark plugs as soon as possible. To correct the problems in the described case, a major engine overhaul is necessary.
Photo #7 shows a spark plug with a completely destroyed central electrode and its ceramic skirt. The cause of such a malfunction could be prolonged engine operation with detonation, the use of fuel with a low octane number, very early ignition, or simply a defective spark plug. The engine operation symptoms are similar to the previous case. The car owner will be lucky if the particles from the central electrode manage to slip into the exhaust system without getting stuck under the exhaust valve, otherwise, a cylinder head repair cannot be avoided.
The spark plug shown in photo #8 has an electrode covered with ASH deposits. In this case, the color of the deposits does not play a decisive role. The reason for this build-up is oil combustion due to wear or sticking of the oil scraper piston rings. The engine shows increased oil consumption and blue smoke is coming out of the exhaust pipe. The procedure for replacing piston rings is described in chapter "Mechanical part of the engine".
It is also recommended to check the condition of the spark plugs during routine vehicle maintenance. In this case, it is necessary to measure the gap between the spark plug electrodes and remove carbon deposits with a metal brush. Removing carbon deposits with a sandblaster can cause microcracks, which can develop into more serious defects, ultimately resulting in the condition described in photo #7. Furthermore, it is recommended to rotate the spark plugs, as the operating temperatures of different engine cylinders may vary (for example, the middle cylinders of engines with central fuel injection operate at higher temperatures than the outer ones).