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The Chevrolet SEO car with a mileage of about 80 thousand km in 2011. Users reflect: the car engine jitter. Inspection and analysis: the maintenance personnel inspect the injector according to the experience and find that the 3 cylinder fuel injection volume is too large. After replacing the 3 cylinder fuel injector, idling at the idle state. Slow rising speed will feel like intermittent fire. The engine control unit is detected, with 4 cylinder faulty code. From the data we can see that the number of 4 cylinders missing fire continues to accumulate. Try to remove the cylinder test by pulling out the high voltage line, and the 4 cylinder did not find any abnormalities. When the 2 cylinder high voltage line is removed, the speed fluctuation is not obvious. At the same time, the 2 cylinder high voltage line was disconnected, and the data of fire deficiency was checked. It was found that the 2 cylinder of the diagnostic instrument not only showed no fire missing data, but the fire data of the 4 cylinder increased obviously. It appears that the missing cylinder in response to the data is misleading, indicating that the order of the misfiring cylinder is inconsistent with the actual situation. Check the spark plug. There is no exception except for the 2 cylinder spark plug. The spark plug has no crack and electricity leakage, and the resistance is 5.4 K ohm, normal. The spark plugs of each cylinder are used for jumping test respectively, and all of them can jump normally. So we tried to replace the ignition coil and the high voltage line, but the trouble remained. Related information: in 2012, Chevrolet Sao original factory maintenance manual from the perspective of the phenomenon, the fault is very similar to intermittent fire. Since the ignition coil and the high voltage wire have been replaced, only spark plugs are left. Although the spark plugs looked good, they decided to replace 4 spark plugs. At this time, the jitter vanished. When the spark plug is re examined, the focus is placed on the 2 cylinder spark plug, which is blackened by the appearance of the spark plug. The skirt of the ceramic body under the central electrode of the spark plug is obviously black, and the normal should be brown or light yellow. Poor ignition spark plug ceramic body temperature is low, can not achieve self-cleaning, so carbon black. The spark plug is re installed on the 2 cylinder, and the jitter caused by the lack of fire appears again, and the data of the 4 cylinder fire is reappearing in the data. The continuous spark test of the spark plug is repeated and it is found that there is intermittent fire. Careful observation shows that there is a leakage spark at the ceramic skirt of the spark plug center electrode. It seems that the leakage of the insulator on the skirt is the reason for the breakdown of the spark plug. Through this car, it is found that some places are easy to cause misdiagnosis. (1) when the ignition coil is controlled by a diagnostic instrument, the ignition coils of some cylinders can not be driven, such as the 2 or 3 cylinders of the car, which can not be driven by a fire, and the 1 and 4 cylinders can drive a fire, which is easily misjudged to be a ignition coil fault. Therefore, when the engine runs or starts the fire test for the demon (2) engine running, the ignition coil shell and the iron lap will appear between the electric spark, similar to the leakage. Compared with the model, the phenomenon is the same. Do not misjudge the leakage of the ignition coil. (3) the cylinder order of the non ignition data is not consistent with the actual situation. Therefore, when the fire is diagnosed, the correct cylinder order should be found through practical testing first, so as to avoid misjudgement. (4) the method of the spark plug inspection is: look at the color, the skirt color can reflect the good or bad of the ignition or the working condition of the cylinder; look at the situation of the continuous jump fire, judge whether there is intermittent fire; see if the skirt of the spark plug is leakage; check the spark plug resistance. (5) the leakage of the spark plug in the long oil flooded carbon deposit will result in the leakage of the skirt. This is also a reason for the decrease of spark plug life. Troubleshooting: replacement of injector and spark plug, troubleshooting.