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An easterly wind car with a mileage of about 75 thousand km and a 491E engine. Users reflect: the engine's idle speed jitter is obvious, the exhaust pipe has abnormal sound, and the tail gas has strong gasoline flavor. When the vehicle runs, it accelerates slowly and even extinguish. After receiving the car: the diagnostic system is used to diagnose the engine system, and the result is normal. A basic inspection of the engine showed that the gap between the spark plugs was too large. Replace the spark plug. Cleaning fuel system and throttle assembly, trial run, failure still. The data flow of engine data is read, and the signal data of the absolute pressure sensor and oxygen sensor of the intake manifold are found, and the data of ignition timing, injection pulse width, idle speed valve and other data are not normal. The vehicle is equipped with a direct ignition system consisting of two ignition coils. The ignition system can not be adjusted at any time. The absolute pressure of intake manifold is 75kPa at idle temperature of engine. Refer to the maintenance manual. The absolute pressure of the intake manifold at idle speed should be 35 ~ 51kPa. Obviously, the intake manifold of the car has absolute high pressure. It is suspected that the leakage of the intake system is checked by the vacuum tube of the brake vacuum booster tube, the carbon tank solenoid valve vacuum tube and the fuel pressure regulator, and there is no leakage problem. No leakage was found in the intake manifold, injector and throttle gasket with carburetor cleaner. Check the intake manifold absolute pressure sensor, the sensor has three lines, namely 5V power cord, ground wire and signal line. Using the multimeter to measure the voltage between the power line and the grounding wire is 5V; starting engine, the signal line voltage of the idle speed is 3. 76V, higher than the normal value; the engine is closed, the signal line voltage is 4.96V. The absolute pressure sensor of the intake manifold is normal, and the fault should be caused by mechanical problems. Opening the valve chamber cover, it is found that the timing belt has jumped two teeth, and the timing mark of the camshaft pulley lags two teeth. Readjusting the timing mechanism, commissioning, and thoroughly eliminating the faults. The timing error of the gas distributor leads to the excessive intake pressure of the intake manifold, which determines the injector, which is bound to cause excessive fuel injection, the engine is not working properly, and the oxygen sensor displays high voltage. The maintenance experience shows that the abnormal intake pressure of the intake manifold is often related to the mechanical problems. The emphasis should be on checking whether the valve timing is correct, the intake system is leaked and the exhaust system is blocked.