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A 2008 Wuling minicar, carrying a B12 engine with a mileage of about 60 thousand km and B12 engine, has a failure to speed up after changing the wire harness in other repair plants, with a maximum speed of only 40 km/h. At that time, the maintenance personnel used the fault detection instrument to test, no fault code, suspected that the fuel pressure is insufficient, but the replacement of fuel pump after the test, the failure is still, so the author for help. After receiving the car: trial run, the engine idle speed is steady, and the engine accelerates in place, the engine responds slowly and the voice is dull. Open the engine room cover inspection and find that the ignition coil, the cylinder line and the spark plug are all new. The fuel pressure is measured from 350 kPa to 380 kPa, normal. There is no fault code with the fault detector; the engine data flow is read, and the idle speed data are not obvious. The rapid acceleration test shows that the ignition advance angle is not obviously ahead of the engine after the engine speed rises, showing 1 to 2 degrees, and it is concluded that the cause of the fault of the car is the abnormal ignition advance angle; read the detonation transmission. Sensilla data flow, it is found that the engine control unit has been able to receive the knock signal, so the ignition time is delayed. The main reasons for engine detonation are the use of inferior fuel oil, engine timing error, high engine temperature, excessive carbon accumulation in the combustion chamber, mechanical vibration interference, etc. Ask the driver to know that the car has been adding No. 97 unleaded fuel at regular gas stations and does not have the possibility of poor quality fuel; the engine is stable at idle speed and can exclude the possibility of engine error; the engine data flow shows the temperature of the coolant in the normal range, the possibility of overheating the engine temperature; the engine fixed support The engine is not aged and has no distinct noise inside the engine, eliminating mechanical vibration interference. The cylinder head needs to be removed because of checking the carbon deposit in the combustion chamber, so it will not be considered for the time being. The car changed the wiring harness. Would it be a wire harness? Combined with the circuit diagram check, it is found that the wiring harness is OK. Reorganize the maintenance idea and reduce the fault point in the coke sensor of the knock sensor, the engine control unit and the combustion chamber. It is unwise to dismantle the engine blindly, so I decided to start with a simple one. In the past, the judgment of this kind of fault is easy to disassemble, and the replacement method is considered. But the requirement of the detonation sensor is very strict and the tightening torque of the bolt is required, so the author has used the oscilloscope. Starting the engine, according to the use of the oscilloscope to meet the probe, the test found that the detonation sensor work normally, no detonation signal at idle speed, rapid acceleration has detonation signal, but at this time the engine control unit has been receiving the detonation signal. Because the oscilloscope probe is measured directly from the terminal of the engine control unit, the breakdown of the detonation sensor and its circuit is excluded, and the damage of the internal circuit of the engine control unit is deduced. Troubleshooting: when the engine control unit is replaced, the engine is accelerated and the speed of the engine can reach 100 km/h easily; the engine data flow is checked and the ignition advance angle is 10 to 20 degrees after the speed rises, and the fault is excluded.