A Audi A8 car with mileage of about 50 thousand km, equipped with V8, engine (4. 2L) and automatic transmission. The malfunction of the vehicle is that the engine's idle speed jitter is obvious, the low speed zone is not accelerated, and the exhaust pipe is fired, but the operation of the high speed zone (3000r/min) is basically normal. After receiving the car: the engine was basically checked and no abnormalities were found. Replace the fuel filter, test the car, the failure is still. The diagnostic system is used to diagnose the engine system automatically. There is a fault code 16486, meaning that the air flow sensor signal is too small. Look at the data flow, and the air flow signal is unstable, which may be related to the unstable operation of the engine itself. Replace the air flow sensor, remove the trouble code, and the trouble does not improve. Again, there is no fault code, and the air flow data are basically the same as the original ones. Check the air filter to the intake manifold without loosening. The intake manifold is injected by carburetor cleaning agent. The engine responds well, indicating air leakage. After careful examination, it is found that the intake manifold pad is aging and fractured. Replace the intake manifold pad, and the failure is completely eliminated. The most obvious feature of this case is that the low speed zone is not speeded up and the speed is normal. This indicates that the fault has nothing to do with fuel pressure and ignition timing. Idle jitter is often related to the abnormal mixture, and the generation of fault code 16486 is related to the leakage of intake manifold. The jet carburetor cleaning agent is a leak detection method. The flammable solvent enters the cylinder to participate in combustion, and the engine responds obviously, so it can prepare the leak spot.