A driving range of about 156 thousand km, carrying a 1.6 L engine with a continuous variable valve timing control system (CWT) and a 2009 Beijing modern pleasure car with a A4CF1 3494 gear automatic transmission. Users reflect: the car has been overhauled in a comprehensive maintenance enterprise due to the oil burning, but the engine has been inadequately powered and jitter after the overhaul, and the engine fault alarm light will be lighted when the engine fails. Because the comprehensive maintenance enterprise does not have the testing ability, the customer will send the fault vehicle to the company where the author works. Inspection and analysis: the author carried out the trial run after receiving the car and found that it was basically consistent with the failure phenomenon described by the customer. So I connect the HI-DS fault diagnosis instrument to try to read the fault code in the fault memory of the engine control unit, and find out 1 fault codes through check: P0300 - any multiple cylinder fire detection. The author uses the HI-DS fault diagnosis instrument to try to clear the fault code and finds that the fault code can be cleared. However, after a certain distance, the engine fault alarm lamp will be lit again, and the fault code will also be found in P0300. In repeated trial run, it is found that the output of the malfunctioning vehicle is quite different from that of the normal vehicle. So the author analyzes and detects the trouble code of "P0300 - any cylinder misfire detection". The fires mentioned here are caused by poor ignition, inaccurate fuel metering, poor compression, or a number of other causes that cause incomplete combustion in the cylinder, even if the amount of fire is small, it will increase the emissions of the mixture because of the insufficient combustion of the mixture. The increase of misfire rate will damage the catalytic converter of three yuan. The engine control unit (ECM) monitors the change of crankshaft speed to confirm whether there is a fire. ECM identifies the specific cylinders that have been burned, and calculates individual fire situations by monitoring the rotation changes of each cylinder crankshaft. Random fire indicates that 1 or more cylinders are lost. If the fire value exceeds the limit, the engine control unit will record the fault code of "P0300 - multiple cylinder fire detection". Through analysis, the author believes that the main causes of the above failure are as follows: bad wiring harness, problems in ignition system, problems in fuel system, problems in the inlet and exhaust systems, and no injector plugging at ignition timing. So the author used the HI-DS fault diagnosis instrument to read the data flow of the engine control system and find the exception through the data flow. The fuel injection time of the fault vehicle at idle speed is 3.1 MS, while the normal vehicle injection time is 2.5 ms. This phenomenon indicates that the fuel injection time of the fault vehicle is too long. According to the P0300 fault code, the fault vehicle is a multi cylinder fire instead of a cylinder fire. Is it caused by fuel supply or blockage in intake and exhaust system? So the author took questions to clean the fuel supply system and the intake and exhaust system. The author removed the spark plug for inspection, and found no serious carbon deposition or leakage. In order to save time, the author directly removed the ignition coil and spark plug from the normal vehicle to install on the defective vehicle, but the failure phenomenon remained. Therefore, the possibility of ignition system is basically eliminated. Since the malfunction vehicle has been overhauled in the comprehensive maintenance enterprise before, the author suspected that it caused mechanical failure or improper assembly during the previous maintenance process. As the vehicle is equipped with the CVVT system, the assembly of the system has caused many similar failures. So the author carefully checked the crankshaft and camshaft timing mark, found that they are within the normal range. Because the 2 camshafts of the malfunctioning vehicle are driven by the timing chain, I wonder if it is caused by chain malocclusion. When I removed the valve cover, I found that the camshaft was wrong at the right time. There were almost 2 teeth, so the cause of the failure was almost certain. The author removed the exhaust camshaft to check whether the work of the CWT assembly within 200 was smooth and found that the work was normal. So the author fixed the CVVT assembly at the maximum angle of the delay angle and locked it, restarted the engine after the timing correction, and found the acceleration and stability of the vehicle during the test. Troubleshooting: after correction, the author uses the HI-DS fault diagnosis instrument to read the data flow of the engine control system, and finds that the injection time is kept in the normal range of about 2.5 ms, and the engine fault alarm light is no longer lighted. At this point the fault is completely eliminated. Review and summary: the camshaft actuator is not fixed at the maximum delay angle when the fault vehicle is overhauled in the comprehensive maintenance enterprise. The camshaft actuator is fixed in the other position, resulting in the deviation of the gas distribution phase. Especially when the vehicle speeds up, there is no corresponding intake advance angle, which causes the engine power to drop.