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In the domestic sports utility vehicle market, some vehicles use electronically controlled common rail diesel engines, such as the Great Wall HAV, Zhengzhou Nissan and Huatai Santa Fe. The BOSCH EDC16 version is widely used in diesel engines. According to the type of high pressure oil pump, it can be broadly divided into two types: Cp1 and CP1 H. In actual maintenance, the diagnostic steps of the gasoline electronic control system are fully applicable to the diagnosis of diesel electronic control system, which is divided into the following steps. (1) read and analyze the fault code. (2) observation and analysis of data flow. (3) component test. (4) circuit test. (5) waveform analysis. (6) fault confirmation. The following is a combination of maintenance cases to illustrate the application of the above steps. The function of fault code in the diagnosis of automobile electronic control system is irreplaceable. Although its quantity is not as much as we expect, we can find out the trace of the fault from the existing fault codes, determine the fault range, and play a guiding role in the fault diagnosis. Fault phenomenon: a the Great Wall Haff sports utility vehicle, equipped with a 2.8 L turbocharged diesel engine, adopts BOSCH EDC16C39 electric control system and CP1 H pump. It is difficult for users to start after parking for some time. Inspection and analysis: the maintenance personnel tried the car and found that the fuel injector did not work when the starter was running. Detect engine control unit and find 8 fault codes. For the first read fault code, do not rush to delete, but should be recorded before deleting. This is because many fault codes are generated by the accumulation of faults for a long time. If they are deleted, it will take a long time to restore them. In the analysis of the fault code, it is also necessary to distinguish between the primary and secondary. When the fault code is recorded and deleted, only P0670 appears when it is read again. However, the reason for the difficulty of starting the vehicle can not be the reason, because the external temperature has reached 30 degrees, relative fault code P1011 is more noteworthy. The explanation of P1011 in the diagnostic instrument is that the positive deviation of the rail pressure controller exceeds the upper limit. For those who do not know much about the common rail system, this explanation is somewhat vague. In fact, the meaning of this explanation is that the fuel control valve in the common rail system is not properly opened. Let's know first what the rail pressure is. The so-called rail pressure deviation is the theoretical rail pressure minus the actual rail pressure difference, if the value is positive, that is, the positive deviation of rail pressure. If positive deviations exist, the actual oil pressure in the rail can not meet the needs of the engine. The rail pressure sensor measures the rail pressure and reports it to the engine control unit to control the opening of the control valve. When the oil pressure is reduced, the opening of the control valve increases, the duty ratio of the control current decreases, and the current value is reduced on the data flow. The current of the control valve is monitored by the engine control unit. If the rail pressure deviate, the opening of the control valve changes, so as to achieve the target value. If the opening of the control valve has exceeded the specified value in the engine control unit program, the trouble code will be generated, and many vehicles will enter the "pacing" mode. Viewing the data flow and closing the rail pressure, we can see that the actual rail pressure is consistent with the target value. In the common rail system, a closed loop high pressure fuel control system is formed by the common rail pressure sensor and the control valve, and the oil pressure in the common rail is constantly adjusted according to the different working conditions. The appearance of P1011 indicates that there may be insufficient oil pressure when starting. Judging from this, there may be some problems of high pressure partial leakage or low pressure partial fuel supply in fuel system. When engine starts, observe data, rotate speed is 160 r/min, rail pressure is only 600 kPa, too low. According to the common rail control principle, when the rail pressure is less than 16 MPa, the engine control unit will not drive injector fuel for protection purposes. At the end of the analysis, the fault is obvious. The problem of oil circuit must be solved first. After cleaning the fuel pressure limiting valve and the fuel filter on the pump and replacing the new battery, the engine starts smoothly. Troubleshooting: checking the amount of oil back in the 4 injectors is large, indicating that the valve components inside the injector have been worn out, resulting in excessive fuel injection, which further causes the slow start of the rail pressure when starting, causing difficulty in starting. The customer showed that the diesel filter was not replaced in the vehicle within 50 thousand km. After the injector is repaired, the vehicle is restored to normal.