A BMW 320i car with mileage of about 90 thousand km, equipped with M43 engine and automatic transmission. User response: when the vehicle starts in cold state, the engine is prone to stall. Therefore, the accelerator pedal must be stepped down before the engine can run. Such faults are usually related to the fuel system. Connect the fuel pressure gauge to start test, the fuel pressure is above 350kPa, normal. Step on the accelerator pedal, start the engine, release the accelerator pedal, and the engine jitter is serious, which indicates that there is a cylinder fault. Remove the 6 injectors, wash them with ultrasonic cleaning apparatus, and install them before commissioning. Replacement of spark plugs and fuel filters has no improvement in fault symptoms. The connection diagnostics is self diagnostic, and the engine system has no fault code. Looking at the data flow, the air flow value is low. Replacement of air conditioning flow sensor, the failure is still. After repeated tests, it is found that during the acceleration process, the abnormal sound is occasionally emitted under the gas manifold. The cleaning agent is injected below the intake manifold and the engine speed changes, which indicates that there is a leakage phenomenon. Check the pipeline below the intake manifold and find that the shell of the crankcase exhaust ventilation diaphragm has cracks. Replace the exhaust box of crankcase and remove the trouble thoroughly. During the operation of the engine, the pressure in the crankcase should maintain a certain pressure difference with the atmospheric pressure. For this purpose, a crankcase exhaust ventilation diaphragm box is set up, one side of the inner diaphragm is communicated with the atmosphere, and the other side is communicated with the exhaust pipe covered by the crankcase and the valve chamber. If the membrane box is damaged, this problem arises: when the engine is running at idle speed, the air pressure in the crankcase is low, the diaphragm breaks through the air flow sensor and enters the intake manifold, causing the trouble of starting difficulties, idling jitter, and flameout easily, when the intake manifold vacuum degree is reduced. The amount of air leaked from the diaphragm will be relatively low, so this kind of fault will have less influence on accelerating the soil condition.