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A car with a mileage of about 90 thousand km and a F20 118I powered BMW 118I car equipped with N13 engines. User response: the idle speed fluctuates seriously after starting the cold car. Inspection and analysis: the maintenance personnel tried the car and found no fault. Check the engine control unit, no fault code. In order to simulate the condition of the failure, the car is put on trial for a period of time until it is fully cooled. Before the engine temperature rises, the data flow of the engine is observed quickly, and the engine load rate is constantly changing, and the cause of this change is the changing torque request of the generator. The voltage of the battery is measured, and the amplitude of the fluctuation is large. After the flameout, the storage capacity of the battery is measured. This indicates that the battery of the vehicle has exceeded its service life. Troubleshooting: after changing the battery, start the vehicle and keep the idle speed steady. The output voltage of the generator is measured, and it is normal. The dormancy current of vehicle is measured within normal range. Troubleshooting.