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A BMW 520i car with a mileage of over 200 thousand km, equipped with M52 engine and automatic transmission. User reflection: it is difficult to start the engine, especially during the start of the hot car, it takes several times to start the car, and the engine acceleration is not smooth enough when the vehicle runs. After receiving the car: connect the diagnostic instrument for self diagnosis, the engine system has a fault code, the content is "44, the fuel tank ventilation solenoid valve wire break". Clear the fault code, the fault code can not be removed, indicating that the fault is real. The fuel tank ventilated solenoid valve, the carbon tank solenoid valve, is installed in the front of the intake manifold and provides the working power by the DME relay. The control terminal of the solenoid valve coil is connected with the 42 terminal of the engine control module. The engine control module uses the pulse signal to control the opening of the fuel tank ventilated solenoid valve. Fault code 44 is related to the fuel tank ventilation solenoid itself. Connect the test lamp to the two terminals of the online beam plug, start the engine, and the test lamp shines, indicating that the connection is normal. Measure the resistance of the solenoid valve in the fuel tank, infinitely large, indicating the opening of the electromagnetic coil. Replace the fuel tank ventilated solenoid valve. Maintenance experience shows that the difficulty of starting a hot car is usually caused by poor fuel pressure. Connect the fuel pressure gauge and start the engine. The fuel pressure is kept at 380kPa at idle speed, and the fuel pressure is maintained well during acceleration. Pull off the vacuum tube on the fuel pressure regulator and smell the vacuum orifice. There is a strong smell of petrol. If you tap the fuel pressure regulator lightly, you can see that the fuel pressure regulator has spilled gasoline. When the engine is turned off, the pointer of the fuel pressure gauge drops quickly, indicating the leakage of the fuel pressure regulator. Replace the fuel pressure regulator and remove the trouble thoroughly. The fuel pressure regulator and the fuel tank of the BMW engine are all vulnerable components. Compared to the fuel - ventilated solenoid valves, the fuel pressure regulator is more vulnerable to damage, and the damage to the engine is greater after damage. In actual maintenance work, the fuel pressure regulator should be replaced as long as the fuel pressure regulator vacuum tube has gasoline smell.