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A modern Beijing Sonata 2 car with a mileage of about 50 thousand km. The car owner reflected: the car was flameout after the car. It was dragged into the maintenance plant before being flooded, because the inlet of the engine control unit was corroded, the circuit board was corroded, and the malfunction appeared after the replacement of new parts. According to the maintenance staff of the factory, they used the fault diagnosis instrument Jinde K61 to test the engine electronic control system fault free code, measure the oil pressure of the fuel system to 350kPa, no abnormal flow test of the injector, and two days did not find the cause of the failure. Fault diagnosis: after the maintenance technician receives the vehicle, he uses the special diagnostic instrument to detect the electronic control system of the engine without fault code. When the ignition switch is located in the "ON" position, no exception is observed in the data flow. Taking into account the fault of the vehicle, although the replacement of the engine control unit, but wiring harness plug will not be a problem? So the maintenance technician pulled off the plug of the engine control unit and decomposed it. After careful observation, it was found that the bottom row of connectors had been oxidized and discolourated. After cleaning all the oxidized connectors with rust remover and anhydrous alcohol, handling the control unit's tie line, reloading and starting the vehicle, the trouble remains. According to previous understanding, the engine control unit of the car was bought from the outside, and the maintenance technician suspected that it was not an original plant accessory. Then the engine control unit was installed on other normal vehicles, and it was found that there was no abnormal phenomenon in the car, and it was not the problem of the engine control unit. According to the fault symptom analysis of the vehicle, the causes of the malfunction are as follows: (1) the injector is abnormal. The ignition system itself fails. The ignition of the engine is incorrect. (4) oil pressure regulator. Idle speed actuators. The intake pressure sensor and so on. The flow test for injector is normal, and the possibility of engine control unit problem can be ruled out. The synchronous waveform of the crankshaft position sensor and the camshaft position sensor is also normal with an oscilloscope. The idle motor is forced to be driven by the tester, and the valve can be opened normally (the idle speed motor adopts duty cycle control mode). The oscilloscope is used to record the working waveform of the ignition system secondary, and the ignition waveform of the ignition system is detected. It is found that the ignition breakdown voltage is higher than the normal value, and the ignition system itself has no problem. The main reason for the high ignition voltage is that the mixture is too thin, and the ignition energy needed is high, so the breakdown voltage will be higher than the normal value. It seems that the problem is still in the fuel system. The maintenance technician observed the injection time with an oscilloscope, and found that the injection time was initially 2. 8ms when the vehicle was in the car, and then gradually disappeared. It seemed that the engine control unit did not send out the fuel injection signal. There is ignition signal and no fuel injection signal. If a person can spray a combustible gas from the intake port, it should be able to carry the vehicle. So the maintenance technician will spray the carburetor cleaning agent from the intake port, and the engine can start. Since the engine can be used when the engine is injected into the combustion agent, and the test of the fuel injection has been done, the problem is that the engine control unit is controlling the injection signal. At this time, the maintenance technician decided to analyze the data flow carefully, hoping to find some clues, then put the ignition switch in the "ON" state, and observed the related data of the engine electronic control system by the fault diagnosis instrument, and found that the intake pressure was equal to the atmospheric pressure and the electric pressure was normal. There seems to be no exception to the flow of data. The maintenance technician then started the engine and read the data flow. By observing the data flow, the maintenance technician suddenly found that the intake pressure sensor signal did not change at this time, or equal to the atmospheric pressure, which could determine the damage of the intake pressure sensor. Will the replacement of this sensor solve the problem? Results after the replacement of the intake pressure sensor, the engine was running normally and the trouble was eliminated. Why can't the engine be able to move after the intake pressure sensor is damaged? According to the working principle of the engine electronic control system, it can be seen that the fuel injection of the electronic control engine at the starting point is calculated by the engine control unit based on the engine speed, water temperature, intake temperature and intake pressure. At this point, the intake pressure signal collected by the engine control unit is the default value. After the engine is started, the intake pressure signal will be used as the necessary load signal, that is, the basic letter one we say, the engine control unit will determine the basic fuel injection amount of the fuel system, and then use the other signals as the water temperature signal as the correction signal to control the injection time. If there is no intake pressure signal, the engine control unit will not be able to determine the basic fuel injection volume. Through the maintenance of the car, the maintenance technicians deeply feel that, as a qualified modern automobile maintenance personnel, when studying the working principles of each system on the car, we must conscientiously comprehend every sentence described in this way in order to reduce the curve and solve practical problems in the actual work.