A mileage of about 50 thousand km, equipped with type 54 engine and automatic transmission, chassis type E71 X6 BMW multi-function vehicle. The failure of the car is: speed up the speed of the powerless, the instrument board engine trouble light has been lighted, the central information display display "engine function decline". After receiving the car: the connection diagnostic instrument performs self diagnosis, and the result has a fault code, which is called "2FBF, fuel injection pressure failure". Clear the trouble code, test, the symptoms disappear, but soon again. A new diagnosis is made, and the fault code 2FBF reappears. Looking at the data stream, no exception is found. The fault code is analyzed, and the pressure of high-pressure pump is preliminarily judged. The engine is a direct injection engine. After the fuel is pressed through the high-pressure pump, the fuel injector is ejected directly into the cylinder to burn. If the injection pressure is not reached, the fault code related to the fuel pressure will appear. So the high-pressure pump is replaced and programmed, and then the road test, the result is still a failure. Check the fuel pressure of the low pressure system. Under normal conditions, the low pressure pressure should not be lower than 475kPa, and the low pressure after 10min should not be lower than 375 kPa. Use a special tool to clamp the fuel line to the high pressure pump, then connect the fuel pressure gauge to measure the pressure of the low pressure system, which can reach 500kPa, but the pressure of the low pressure drops to below 200kPa after 10min, indicating the problem of the fuel pressure device. The fuel pressure is integrated in the fuel filter, so replace the fuel filter. Test the car, the fault is completely eliminated. The fuel system of the direct injection engine is divided into two parts: one part is the low voltage system, which is mainly built up by the fuel pump and the fuel pressure regulator; the other is the high pressure system, and the low pressure fuel is pressurized by the high-pressure pump. Whether the high-pressure injection is normal is based on the normal low pressure system. Therefore, the problem of high pressure pump and the low pressure system should be considered when overhauling. Correct inspection order: check the low pressure system first, then check the high pressure system, so that the expensive parts can be avoided.