The mileage is about 130 thousand km, carrying a 2.5 L V6 engine in 2009 Buick GL8 business vehicle. User response: the car is shaking at idle speed but running normally. Inspection and analysis: the commissioning of the maintenance personnel confirms the phenomena reflected by the users. Detect engine control unit and do not find fault code. When the engine idling, observing the data flow, we find 2 exceptions. One is that the long-term fuel injection correction is too large, and the second is that the intake temperature is extremely high. When the idle speed is increased, the intake temperature drops to the normal value rapidly, and the long-term fuel injection correction amount also decreases. This further indicates that the problem is confined to idle state and is consistent with the actual situation. It can be seen from the data of air flow that the long-term correction of fuel injection is too large and has nothing to do with the intake measurement error. According to the experience of data analysis, next should focus on the combustion of mixed gas. The intake temperature of the engine mainly depends on the surface temperature of the parts of the intake system. When the engine speed is low, the heat exchange time of the air flow and the surface of the parts is sufficient, so the temperature is slightly higher. However, the intake temperature difference between high idle speed and normal idle speed should not be higher than 29 degrees. This indicates that there is an abnormal heat source in the intake system. In the intake system, besides the heat of the part surface, the biggest heat source is the exhaust air flow. From the abnormal phenomena of intake air temperature, it is concluded that some of the exhaust air should enter the intake system. If the engine exhaust gas is mixed into the mixture, it will reduce the temperature of the mixture combustion. In this way, as the combustion temperature of the mixture drops, the oxygen consumed in the combustion will decrease slightly, which coincides with the abnormal performance of the injection correction. Read out the opening data of exhaust gas recirculation valve EGR with fault diagnosis instrument, normal. However, as the focus of analysis and judgment falls here, it is decided to remove the EGR valve check. When the EGR valve is removed, the compressed air is blown from the exhaust port to the intake end, and a part of the airflow is ejected from the intake end, which is abnormal. Carefully check the spool, found fouling it can not be closed. Troubleshooting: clean up the EGR valve, confirm the trouble shooting.