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A red flag car with a mileage of about 160 thousand km and a six cylinder AEK engine (2.4L) equipped with 2005 cylinders. User response: the engine can not start when the car is started. After receiving the car: after checking, it found that the timing tooth belt was broken, resulting in engine parts damage. Further investigation revealed that the 1 cylinder and 3 cylinder valves were deformed with the piston and the other parts were not abnormal. After changing the valve, the engine started to start. After 10min, there was a slight noise at idling. The abnormal noise disappeared, but the engine began to shake. Using diagnostic instrument for self diagnosis, when checking the data flow, it is found that there is a continuous fire in the 5 cylinder. Check the air flow and fuel injection pulse width data, basically normal, that is, the air fuel ratio is normal, which also shows that the electronic control system of the engine is normal. The single cylinder fire can only enter the cylinder and the air can not be fully heated, causing the output torque of the cylinder to decrease. Continue to check the long-term fuel correction amount, which is positive, indicating that the oxygen content in the exhaust gas is too high, and the combustion of the 5 cylinder mixture is not enough. The pressure of the cylinder is measured, and the pressure of each cylinder is basically the same. The 5 cylinder fuel injector and spark plug are exchanged with other cylinders, and the 5 cylinder is still on fire. Detecting the injection pulse waveform and ignition waveform of the 5 cylinder is consistent with the other cylinders. Therefore, it is obvious that the 5 cylinder is more likely to have mechanical faults. Re check the gas distribution system, open the valve chamber cover, press the hydraulic tappet, found that the 5 cylinder exhaust valve spring is weak, a little press will press the hydraulic tappet down. Take off the camshaft, take out the 5 cylinder exhaust valve spring, and find that the spring has broken. In order to eliminate the fault thoroughly, replace all valve springs, test engine, idle speed of engine is stable, and maintenance work is over. Rupture of valve spring is the main cause of poor combustion of 5 cylinders. During the testing process, there was no problem in measuring cylinder pressure, because the measurement method was not accurate enough. At the time of measurement, the engine speed is low, and the valve still has time to shut down, even though it is slow down under the action of the broken spring, so it has little effect on cylinder pressure. After the car, the engine speed is higher, the spring is too late to shut down, the mixture can not be fully compressed, so the fuel is bad. In view of this kind of fault, there is a kind of cylinder pressure measurement method, which is to connect the cylinder pressure meter first, then run the engine, carry on the dynamic cylinder pressure measurement, the emphasis is to observe the change of the pointer. This method is a standard test method, and maintenance personnel can be applied more in practical work to avoid misjudgement.