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Case description:

A car with a mileage of less than 20000 km went to the maintenance station for repair. The customer said that the car now uses equivalent gasoline (100 yuan), which reduces the driving distance by about 50km compared with that before failure. At the same time, he found that the exhaust pipe was emitting black smoke; In other words, the fuel endurance is greatly reduced and the fuel consumption is high.

Failure cause analysis:

As it is a car with a mileage of less than 10000 km, the cylinder pressure of the car engine should be normal, so the cause of the fault should be related to the damage of the car brake system and the coolant temperature sensor of Baotou Beiben Heavy Truck accessories, excessive fuel injection of the fuel injector, damage of the oxygen sensor, ignition performance, etc.

Troubleshooting:

After receiving the vehicle, lift the vehicle. After braking, the four wheels do not have brake drag, and the parking brake is normal. Then check the fuel pipeline of the engine, no oil leakage is found, and the circuit is relatively neat. The spark plug spark test after removal is normal. Read the fault code with the X-431 fault diagnosis instrument and detect that the fault code is P0560 system voltage fault. P05050 idle air control stepper motor is short circuited to ground or power. The prompt for the fault code does not think it has much to do with the fault. Focus on checking the coolant temperature sensor and its signal voltage. There is no problem if they conform to the logic of negative temperature coefficient. Remove the fuel injector and conduct a leak test on the vehicle. The inspection result shows that there is no leak. Put it on the ultrasonic detector to check the fuel injector, and the fuel injection quantity is within the normal range. Read the data stream. The oxygen sensor voltage is about 0.9V. If the signal voltage is too high, the mixture is too rich. It is suspected that the oxygen sensor is damaged. Remove the oxygen sensor and measure the resistance of the heating coil to 10 Ω. The circuit of the oxygen sensor is normal within the normal range. It is suspected that the ECU of the engine electronic control unit is faulty, and it is interchangeable with the normal vehicle. After test run, the fault remains. Check the data flow carefully, and find that the battery voltage is 0.51V at idle speed, and the normal value should be between 12.6 and 14.8V. The system voltage is too low, which makes the engine ECU unable to perform closed-loop control on the fuel system. The engine electronic control unit ECU needs three power supply wires to work normally – the battery normal power supply wire, the ignition switch ON shield wire, and the main control relay output power supply wire. After inspection, it is found that the engine electronic control unit ECU lacks the live wire of the main control relay. Check the circuit from the main control relay to the engine electronic control unit ECU. Check that the circuit is normal without open circuit. Check the plug end of the engine electronic control unit ECU. It is found that the plug hole at the plug becomes larger. Baotou Beiben Heavy Truck accessories cause poor contact between the engine harness plug and the engine electronic control unit ECU jack.

Troubleshooting plan:

After processing the engine harness plug and the ECU end socket of the engine, check that the data flow returns to the normal value, troubleshoot the test run, and ensure that the exhaust pipe does not emit black smoke. After a week's telephone call, the customer reported that everything was normal and the fault was eliminated.

Fault summary:

Modern automobile is a microcomputer controlled system. The fault phenomenon is not directly related to the real fault cause of the automobile. The fault code read by the automobile fault diagnosis instrument only reflects the fault of the automobile sensor, actuator, electronic control unit and related circuits. The internal control comprehensive fault requires the maintenance technician to use knowledge and experience to analyze and judge the fault cause and location. Conditions for normal operation of the automobile electronic control unit: the power supply is normal, including the normal live wire, and the live wire provided by the ignition switch of Baotou Beiben Heavy Truck Accessories at ON gear and the live wire provided by the main relay; The grounding of the electronic control unit is normal, and the electronic control unit is normal. The reason for this vehicle is that the engine electronic control unit cannot work normally due to an intermittent power supply fault, which causes the engine to enter limp home mode. Fuel injection cannot adjust the injection time according to various sensor data of the engine.