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The mileage is about 28 thousand km, carrying the new 1.6 Fawkes L and 6 speed dual clutch transmission in 2014. The car was jitter to the factory for engine idling. According to the driver, the car will appear the engine idle jitter after the driving time in the city, high speed running without problems, the vehicle driving performance is not affected, the acceleration shift is very stable. Fault diagnosis: after the vehicle is first tested, accompanied by the customer road test run about 1 h, there is no customer description of the phenomenon of failure. Connect the fault detector IDS to read the fault code of the engine system and trouble free code storage. Using IDS to do network testing, all modules are qualified, indicating that the power supply and the grounding of the relevant modules are normal. After asking the customer to know that the car has done the cleaning of the injector and throttle for the fault before the car, the author emphatically inspected the throttle body and its related lines, and no exception was found. Because the customer was unwilling to leave the vehicle to stay in the factory, the author adjusted the throttle body of the vehicle and trial driving, and the client drove away from the factory. Third days in the morning, the car went to the factory again. The engine idle jitter was serious. Sometimes the engine speed dropped to 300 r/min. The engine fault lamp, the battery alarm lamp and the oil alarm lamp were abnormally lit. Using IDS to read the engine system fault code, no fault code storage; read the engine system data flow discovery, the engine intake flow change is larger (Figure 1), under normal conditions, the engine idling intake flow should be about 1.8g/s. According to the analysis of the read data flow and the failure phenomenon, the possible causes of the fault are: the failure of the intake system and its related line faults, the offset or interference of the signal, the fault of the engine control unit, the error of the system program, and the failure of the fuel supply system. First, the calibration of the engine control unit (PCM) is upgraded, and the related lines of the throttle body, the air flow sensor and the injector are checked, all of which are normal. According to the condition that the air flow signal was disturbed before the model, the line of the air flow sensor was shielded and the SP190 hinge joint of the line was soldering. After adjusting the air flow sensor, the failure phenomenon disappeared and the data flow of the engine system was read. The road test was normal. After 20 km, the failure was not reproduced. Because the customer has been repeatedly maintenance, dare not hastily deliver the car to the customer, second days morning again on the road trial run, after running about 0.5 h, the fault reappears. After returning to the plant, connect the fuel pressure gauge to check the fuel pressure, and the fuel pressure is always 390 kPa. Remove the fuel filter and check the fuel quality. The fuel quality is good. After debugging the fuel injector of the vehicle with the pilot injector, the trouble has not been ruled out. Finally, the engine control unit is tested with the normal vehicle, and the trouble has not been ruled out. According to the failure phenomenon and the read of the engine system data flow, the fault should be out on the intake system, and then the emphasis on the intake system has been checked to confirm that the system components do not have problems, so the maintenance is in a deadlock. After careful analysis of the data flow of the engine, it is found that the engine speed is also 0 r/min when the failure occurs (Figure 1C), but the engine does not turn out during the actual operation, and the PCM should not receive the signal of the engine speed of 0 R /min. The idling control logic of the car depends on the intake volume of the throttle. The opening of the throttle depends on the PCM signal. The PCM receives the crankshaft position sensor (CKP), the coolant temperature sensor, the oxygen sensor and other signals, and then regulates the intake volume and the injection amount according to the internal logic. When the car fails, PCM receives the changing engine speed signal, even sometimes the engine speed is 0 r/min. After the PCM receives this speed signal, the adjustment and compensation of the throttle opening will result in the constant change of the intake volume, and logically, the engine speed changes affect the hair. The intake of the motive. The cause of the fault should be the signal of the crankshaft position sensor, and the relevant circuit diagram shows that the CKP signal is sent directly to PCM through 2 wires. Because the car is newly purchased soon, the probability of line failure is very small, PCM has been adjusted with the normal vehicle, so the CKP is checked first. The car's CKP is mounted on the flywheel side. After dismantling the CKP, it is found that the head of the sensor has adsorbed a large number of metal pieces. At the same time, compared to the CKP of the normal car, the car's CKP head has two marks, and the CKP is contaminated with the signal distortion, providing an untrue current engine speed, and PCM after receiving the wrong speed signal. The valve opening, which causes the failure. Troubleshooting: the replacement of the crankshaft position sensor, and the flywheel side, the transmission in the cleaning, many road test failures are not reproduced, a week later to visit the customer to know that the vehicle is normal, so to confirm the troubleshooting.