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A mileage of about 120 thousand km, the vehicle is E60, equipped with N52 6HP-19 and 6HP-19 transmission of the BMW 523Li sedan. Car owners reflect: the engine in idle state of engine speed fluctuates up and down, and the engine yellow fault lights in the instrument light and report glimpse; immediately after the failure to start off again, the phenomenon will disappear immediately, and sometimes the failure phenomenon can also disappear automatically. There are no specific rules for faults. Cold cars have appeared and hot cars have also appeared. Speed is always more normal during the driving. After receiving the car: connect ISID, there is fault code HFM (air flow sensor) too low fault. Read engine operation value, engine speed at 672r/min, air flow rate is only 8kg/h. The normal air flow of this car is about 12kg/h at idle speed. It is obvious that the intake volume measured by the air flow sensor is less. First check the intake line in front of the air filter and the air flow sensor. The air cleaner is clean and no blockage. Check the connection terminals of HFM is not loose, and the line to DME is also normal, no problem. Therefore, the preliminary judgment is HFM fault. After commissioning the HFM with other normal vehicles, the failure has not been eliminated. This is not a HFM problem. The call control module reads the data stream of DME and reads the relevant data stream. Oxygen sensor data: the pre oxygen sensor is 1.2V, the post oxygen sensor is 0.8V, the standard oxygen sensor is 2.0V, the post oxygen sensor 0.7Vo mixed gas addition adjustment value: the mixing gas addition adjustment value is 0.8, the mixed gas addition adjustment value standard value is 1 ~ 1.14, the mixture gas addition method is adjusted to the lambda <1, indicating that the vehicle mixture is too strong. Generally, the over concentration of mixed gases is mainly caused by the following reasons: excessive fuel injection, high oil pressure, too little air intake, and carbon deposit in the engine combustion chamber. According to several reasons, the pressure of the fuel system is 350kPa, normal. When the injector is read, the fuel injection is 2 ~ 3MS, and the cause of the oil supply system is initially excluded. Using the endoscope to observe the combustor, there is no obvious carbon deposit in the combustion chamber. After all, the mileage of the vehicle is not very large. The fault point is returned to the intake system. By reading the fault content and the intake data flow, it is also obvious that the intake is too small, only 8kg/h, far lower than the normal 12kg/h. In a certain case of injection, the engine will certainly cause the engine mixture to be too strong, the combustion is insufficient, the emission exceeds the standard, and the engine idling jitter is caused. Engine trouble light yellow light alarm. Next, find out what causes the amount of air intake to be too small, and the trouble can be solved easily. This requires a general understanding of the intake system of N52. The intake system of the N52 engine uses the VALVETRONIC, the fully variable valve travel control device and the variable camshaft control device (double VANOS), as shown in the diagram. Throttle valve is almost completely opened during the intake process of VALVETRONIC engine, and intake control is realized through the closing time of intake valve. The fully variable valve stroke control is achieved through a servo motor (VVT motor), an eccentric shaft, a middle push rod, a back spring, an intake camshaft and a roller type valve. The servo motor is installed in the cylinder cover above the camshaft, and is used to adjust the eccentric shaft. The worm shaft of the motor is embedded in the worm wheel installed on the eccentric axis, and the eccentric shaft is not specially locked after adjusting, because the worm drive mechanism has enough self locking ability. The eccentric axis regulates the valve stroke on the intake side, and the push rod changes the transmission ratio between the camshaft and the roller type valve, and the valve travel (9.9mm) and the opening time reach the maximum when the load is full. At idle speed, the valve stroke (0.18mm) and the opening time are minimum. The position of the eccentric shaft is detected by the mountain eccentric shaft sensor and sent to the DME. Through the data flow, the eccentric shaft position is 17 degrees, while the normal position of the eccentric shaft is 30 degrees in the normal idle state. The general condition should be 26 degrees. The position of the eccentric axis is nearly half less than the theoretical value. Then it will directly affect the transmission ratio between the middle push rod and the rocker arm of the roller type valve. The opening stroke of the intake valve will be reduced, the air volume of the various combustor will decrease, and the air intake of the whole intake manifold will be reduced accordingly, and the air is measured by HFM. The flow rate is also lower than the normal value, so it causes idle engine jitter and engine emission lamp glimpse. Back to the back analysis, the eccentric axis is driven by the servo motor through the worm shaft to drive the eccentric shaft, the turbine and the eccentric shaft are made into one. In general, there is no problem. The worm and the servo motor are always integrated, and the rotation position (angle) of the eccentric axis will not be the angle of the rotation of the servo motor. Enough? Theoretically, it is possible to analyze it. As a result, the maintenance personnel find the same vehicle, on the one tune servo motor, using ISID to initialize the valve lift, then start the vehicle, the engine jitter disappear. Again, the position of the eccentric shaft is 26 degrees, and all the data of the oxygen sensor are restored to normal value. Troubleshooting: fault is the servo motor rotation angle is not enough, resulting in valve lift is not enough, resulting in insufficient intake air to cause excessive gas mixture fault. After replacing the servo motor, removing the adjustment and learning the minimum lift of the valve, starting the vehicle, observing the data flow of DME, the intake volume in 12.5kg/h, the voltage of the front oxygen sensor 2.0V, the voltage of the post oxygen sensor 0.77V, the mixed gas addition adjustment value of 1.14, clearing the fault memory of the fault memory and troubleshooting. Fault summary: this car tells us that the problem of thinking should be comprehensive and any problem is regular. If we find out carefully, we can make a careful summary and make us solve the problem more quickly and more accurately.