One mileage is about 50 thousand km. The N54 double turbocharged engine is equipped with 2010 BMW 740Li cars of F02. User response: when the vehicle is speeding up, the engine yellow light alarm is lit, and the central information display indicates "engine failure and output power drop". It felt that the vehicle accelerated the performance decline. Fault diagnosis: after receiving the vehicle, the ISID is connected to the diagnostic test, and the related fault content is read "120308- pressurization pressure regulation, lower value; supercharging pressure is too low." The following points are the following: vacuum supply device, pressure relief valve, pressurized air conduit, turbocharger, exhaust back pressure. First of all, general visual inspection is carried out to check the pressure relief device to control the vacuum pipeline, the pipeline is sealed and the installation is correct without bending; the pressurized air pipe and hose from the air compressor to the throttle are checked and the pipe is sealed and installed correctly; the intake pipe to the air compressor is checked, the intake pipe is sealed and installed correctly. Next, check the vacuum pressure of the vacuum device of the decompression device. There are two kinds of pressure relief devices on the N54 engine. One is the pressure reducing device of the exhaust valve. The pressure of the exhaust turbocharger is directly related to the exhaust gas flow to the exhaust turbocharger turbine. Both the speed and the quality of the exhaust gas flow are directly dependent on the engine speed and the engine load. The engine management system regulates the pressurization pressure through the exhaust bypass valve, and the exhaust bypass valve is manipulated by the vacuum actuator. These actuators are controlled by the engine management system through the electronic pneumatic pressure converter (EPDW). The engine vacuum pump generates a vacuum and stores it in a pressure accumulator. Through the exhaust gas bypass valve, all or part of the exhaust gas flow can be transported to the turbine, and when the required pressure pressure is achieved, the exhaust gas bypass valve begins to open and some exhaust gas flow is discharged through the bypass channel, which prevents the turbine from continuing to improve the compressor speed and can handle various operating conditions through this control method. The two is the intake circulation air pressure reducing valve. The N54 engine's circulation air pressure reducing valve is used to reduce the peak pressure peak which is not expected to occur when the throttle is fast closed. If the engine speed is high, close the throttle valve, and the vacuum pressure will be generated in the intake pipe. Because the passageway to the intake pipe has been blocked, a larger back pressure will be formed after the compressor, which will cause the supercharger to pump gas. This means that the disturbing pump noise is clearly sensed, and the exhaust turbocharger also bears the load that can cause damage to the component, because the high frequency pressure wave exerts an axial load on the exhaust turbocharger bearings. The circulating air pressure reducing valve is controlled by an electric switching valve (EUV). According to the running state of the engine, the vacuum pressure of the circulating air pressure reducing valve is controlled by the intake pipe pressure or the vacuum of the vacuum system. EUV is not used in larger operation range. In this case, the circulation air pressure reducing valve is opened through the pressure difference between the intake device and the pressurized air pipe. If there is pressure difference between the throttle valve, the intake pipe pressure will open the circulation air pressure relief valve and turn the boost pressure to the intake side of the compressor. Once the pressure difference exceeds 0.3 bar, the circulation air pressure relief valve will turn on. This process prevents interference pump action that causes damage to components. Even if the engine runs near the idle speed (P pressurized / P inlet pressure difference = 0.3 bar), the circulating air pressure relief valve will be opened according to the system requirements, but will not further affect the turbocharging system. The exhaust turbocharger is subjected to all the exhaust gas flow pressure in these low speed ranges and is close to the idle speed. A certain pressure is imposed on the intake in advance. If the throttle valve is opened at this time, it will quickly provide all the booster pressure needed for the engine. The function of the N54 engine vacuum system is trimming, the vacuum pump produces the vacuum for the brake booster and the vacuum used to control the exhaust valve, and the circulating air pressure relief valve is now vacuum through an electric conversion valve (EUV). The vacuum accumulator is integrated in the engine cover plate. Vacuum accumulator can provide vacuum for the control of exhaust bypass valve continuously. There are three vacuum lines connected to the engine cover plate. One of the pipelines is used to provide vacuum generated by the vacuum pump, and the other two pipelines are used to control two exhaust by-pass valves. Remove the vacuum tube of the exhaust valve to measure the vacuum pressure. This vacuum is used for exhaust valve control. The idle speed is - 0.9bar, and when the load is loaded - 0.38bar, normal. Remove the vacuum pump to the vacuum pipeline of EUV to measure the vacuum pressure. This vacuum pressure is used to control the intake air pressure relief valve. The idle speed and acceleration are all - 0. 9 bar - right. The vacuum tube vacuum of the vacuum pump is about - 0. 9 bar. These two values are normal. Then the vacuum pressure after the throttle is measured. 0. 6 bar, continue to measure the vacuum pressure from EUV to circulating air pressure reducing valve. The result is "0". It is stated above that the EUV can not play a role in the larger scope of operation. The circulating air pressure relief valve is directly controlled by the pressure in the intake manifold. The connection of the intake manifold side of the EUV and the air pressure reducing valve side of the intake air is directly connected, and the electronic control part only controls the pressure of the vacuum pump, that is, EUV to the circulating air. The vacuum pressure of the pressure reducing valve should be about 0. 6 bar as the pressure in the intake manifold. But when measuring vacuum pressure in the vacuum pipeline of vacuum pump to EUV, it is clear that there is an interface with vacuum pressure in EUV. Is the vacuum pipeline connection wrong? Quickly find a similar vehicle for comparison, it is indeed a vacuum pipeline connection error. Vacuum pump to EUV vacuum pipeline connects to EUV to circulating air pressure relief valve end, but connects EUV to vacuum pump end to the vacuum pipeline of the circulating air pressure relief valve, so the vacuum pressure of the vacuum tube of the circulating air pressure relief valve will be displayed as "0". The normal connection should be shown in Figure 2. This is a fault caused by an artificial installation. After the two pipe tubes are installed, the vacuum pressure produced by the vacuum pump is directly connected with the intake manifold, causing the vacuum pressure of the vacuum pump to decrease in some conditions. The bypass valve of the exhaust turbocharger is controlled by the vacuum pressure produced by the vacuum pump. After the pressure drops, the bypass valve is delayed or not completely closed, which affects the speed of the exhaust turbine and further causes the pressure to be too low. Re connect the vacuum line of EUV correctly, trial run, troubleshooting.