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A BMW 740Li car with a mileage of about 60 thousand km, a N54 engine and an automatic transmission, and a chassis model of F02. Users reflect: the vehicle speeds up during the speed of the powerless, the instrument board engine light light light, the central information display display "engine power decline, please go to the BWM maintenance station check." For this reason, replace the fuel pump and pressure reducing valve to adjust the solenoid valve, but the trouble is not ruled out. After receiving the car: connect the diagnostic instrument for self diagnosis, the fault information is too low pressure. To find out the condition of the fault information, the engine control module monitors the pressure and compares it with the standard value. If the measured pressure is lower than the standard value 9kPa, and the pulse load parameter is too large at the same time, the fault is identified and the fault code is set. The pressure sensor is damaged or malfunction, the pressure converter of the pressure reducing valve is damaged, and the pressure reducing valve is damaged (at the open position). The pressure relief valve refers to the bypass valve on the turbocharger, which is driven by the engine control module through the solenoid valve to control the vacuum pressure. If the pressure relief valve is difficult or damaged, this kind of failure will occur. Therefore, the test module menu is used for driving test. It is found that the push rod of the pressure reducing valve can normally expand, indicating that the pressure reducing valve itself has no problem. Vacuum gauge is used to measure the vacuum degree of the pressure converter inlet (-90 ~ 80kPa). The actual test results show that the vacuum degree is obviously lower than the standard value. Check the vacuum line, there is no leakage or blockage. Replace the vacuum pump, test the car, the fault is completely eliminated. The N54 engine is equipped with an electronic valve control system. There is no vacuum inside the intake manifold. Therefore, an independent vacuum pump is required to produce vacuum pressure to achieve turbocharging and braking function. The pressure reducing valve is driven by the vacuum pump. If the vacuum pressure is insufficient, the opening position of the pressure reducing valve can not reach the specified value, and the boosting pressure will be affected. At this time the engine control module will enable the emergency program, although the vehicle can drive, but the acceleration of the turbocharging effect is affected, and there will be fault warning information. Therefore, after determining the pressure reducing valve and vacuum pipeline no problem, it can be judged that the fault is caused by excessive wear of the vacuum pump.