A Audi Q5SUV with a mileage of about 36 thousand km, an engine type of CAD and a transmission type of 0B5. Car owner reflects: the car idling when the jitter is obvious. After receiving the car: 5052 test engine 1 cylinder idle fire obvious. The ignition coil of the dismantling spark plug is normal, and the dismantling intake port finds that the coke accumulation in the intake valve is serious, cleaning the carbon deposit and changing the position of the cylinder nozzle and the 4 cylinder injector nozzle. After cleaning the car, the car failure is still still, the 1 cylinder is still on fire continuously, and the fire phenomenon is slight in the accelerated state. It is normal to remove the engine computer to the ignition coil and the injection nozzle connecting wires, and the computer failure of the replacement engine still exists. It is concluded that the phenomenon of misfire is only evident at idle speed, and the phenomenon of misfire is very slight at high idle speed. After the customer agrees, the cylinder head is decomposed and the valve spring is replaced. After the vehicle is caught, the fire failure is gone, and the engine operates normally. The analysis may be that the spring tightening capacity is problematic, replacing all the springs of the inlet and exhaust doors, excluding the faults (the valve is closed for a long time so that the gas gate is ablated, and the cylinder head assembly should be replaced).