A BMW 520i car with mileage of about 180 thousand km, equipped with M52 engine and automatic transmission. User response: after starting the vehicle, the engine's idle speed jitter is obvious. The air flow sensor has been replaced, but the trouble remains. Measure fuel pressure, fuel pressure is 300kPa, normal. Replacing spark plugs, engine performance has improved slightly. The cylinder breaking test showed that 3 cylinders and 5 cylinders were not working well. It is normal to measure the cylinder pressure of these two cylinders. Remove 6 injectors and check with injector detector. All fuel injectors have normal fuel injection conditions. Check the wiring connection of the injector. It is found that there is a terminal skew in the wiring harness plug of the 3 cylinder injector. After handling the test, the 3 cylinder working phenomenon is gone. Check the exhaust condition, smell is very strong, pull out the oil dipstick, found that the oil is gray, indicating that the coolant mixed with oil, the cylinder gasket may be damaged. Remove the cylinder head and find that there is a rust on the 5 cylinder sealing ring. The water channel is connected with the combustion chamber. Replace the cylinder gasket, install the relevant parts, test the car, and eliminate the trouble thoroughly.