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Shanghai Volkswagen Santana GX with a mileage of over 200 thousand km. Although the engine can start up, it runs unstable under idle condition. Troubleshooting: (1) cause analysis of the cause of engine idling instability, there are many reasons, air supply system is blocked or leak, fuel supply system oil supply is not smooth, ignition system is lack of fire or spark weak, ignition timing is not correct and so on can lead to engine idle instability. (2) troubleshooting method check carburetor oil surface, in the normal range; check the intake system, no blockage and air leakage; check ignition timing, in line with the regulations; high pressure test method to check high pressure spark, found that sparks are obviously weak. The reasons leading to the weak spark include ignition coil and low voltage circuit and components and circuits of high voltage circuit. The resistance of the ignition coil is checked, the resistance of the primary winding and the resistance of the secondary winding are within the normal range, and the connection of the low voltage line is not abnormal; the failure of the replacement of the new ignition coil remains. Therefore, it is suspected that the fault is caused by the high voltage circuit outside the ignition coil. It is found that the resistance of the central high voltage line and the high voltage lines of each sub cylinder is much larger than the normal value, indicating that the high-voltage wires are all aging. (3) after the replacement of the new high-voltage conductor, the failure phenomenon of the unstable idle speed is eliminated and the trouble is eliminated. Fault analysis: the ignition system of Santana GX car engine in Shanghai has damping resistors on high voltage wires, which are used to suppress radio interference. The damping resistance of high voltage wire is mixed with carbon and fiber, and the resistance of high voltage wire will increase after aging, which makes the ignition energy of high voltage loop (spark plug electrode jump) increase correspondingly, resulting in the weakening of the spark, the drop of ignition rate, the increase of fuel consumption and the instability of the idle speed of the light engine; the heavy, launched. The machine can't start. Sometimes the resistance of the high voltage wire will be within the normal range, but the leakage caused by the damage of the insulation layer of the high voltage wire may cause the failure of the engine starting and the idle speed. Therefore, when inspecting the high-voltage conductor, we should not neglect the inspection of whether the high-voltage conductor is damaged.