故障现象一辆行驶里程约为21万km的2005年产别克GL8车,车主反映,最近该车冷起动后怠速不稳,发动机抖动5 s左右并伴有放炮现象,发动机转速最低时才300 r/min,但发动机转速一旦上去后发动机便运转正常。故障诊断对故障车辆进行试车,并采集相关发动机数据进行分析。每次早晨冷起动后发动机转速上下波动持续5 s左右,逐渐恢复正常,熄火后第2次起动车辆无异常现象。用水冲洗发动机3 h以上再次起动车辆,故障重现。观察发动机数据,发现发动机怠速步进电动机的开度很大(为248计数),如图1所示;利用TECH2手动调试降低怠速步 Symptoms A mileage of about 210000 km Buick GL8 2005 car, the owner reflects the recent cold idling after the car was unstable, the engine jitter about 5 s and accompanied by blasting, the lowest engine speed only 300 r / min, but once the engine speed up the engine will run normally. Fault diagnosis The faulty vehicle test run, and collect the relevant engine data for analysis. After cold start each morning, the engine speed fluctuated up and down for about 5 seconds, gradually returned to normal, and the second start after the flameout had no abnormality. Rinse the engine with water for more than 3 hours and start the vehicle again, the fault is reproduced. Observe the engine data and found that the engine idle speed stepping motor opening is very large (248 counts), as shown in Figure 1; manual tuning with TECH2 reduce idling step