The TOYOTA land cruiser with a mileage of about 190 thousand km is 4700. The engine is idle, but the engine doesn't vibrate when it turns on the air conditioner. Fault diagnosis: after the car, the vehicle is checked according to the customer's description, starting the engine, closing all the electric equipment, the engine is jitter at idle speed, the engine fault light does not light, when the air conditioner is opened or the engine speed is raised to more than 800r/min, the motive jitter vanishes. The possible causes of the fault are as follows: ignition system failure, intake system failure, fuel system failure, engine mechanical failure. First, GTS is used to check the engine system, and it is found that there is a irregular fire phenomenon between the 1 cylinder and 8 cylinder when the engine is idle. When the engine speed is raised to 800 r/min or the air conditioning is opened, the phenomenon of fire loss disappears. It is doubted that the intake system or the fuel system is overaccumulated, and the fault remains after cleaning the throttle and the injector. Check the ignition system to find that the spark plug is not the original accessories, so the spark plug is replaced after the replacement of the spark plug; the injector is dismantled and tested by the test platform, the fuel injection is good in each cylinder. It is normal, and the pressure of each cylinder is normal. The maintenance is in trouble at the moment. The customer was asked to know that the car had an accident a long time ago, causing the damage of the engine cover, the damage of the condenser, the damage of the timing belt, but the timing belt was not broken, and it was thought that the fault could be caused by incorrect timing. Remove timing belt cover and observe timing mark. It is found that the timing mark is not correct (Fig. 1). Troubleshooting: replace the timing belt of the engine, re check the valve timing, and troubleshooting.