This low-speed truck generally has the following two characteristics: first, there is always air in the pipeline, and the two is that the engine is powerless. The main reason for this strange fault is the serious wear of the injector needle coupling. There is a low speed truck with a S1110 diesel engine, which is basically normal on a flat road and up the slope, but the diesel engine is automatically extinguished as long as the oil throttle is reduced on the downhill. The occurrence of this phenomenon is caused by the intake of air at the junction of the oil pipe connections. However, after replacing all gaskets at the oil pipe connections, the trouble can not be ruled out. After repeated inspection, it was found that the needle and valve parts of the injector were seriously worn out. When the low speed freight car was driving and uphill in the flat road, the gas in the cylinder could not enter the injector because of the large oil throttle and the high pressure fuel in the high pressure oil road. But when the vehicle downhill, the high pressure oil road is basically in the state of stopping the oil supply due to the decrease of the oil throttle, so the high pressure fuel in the cylinder is added into the pin valve couple, and the low pressure oil is entered into the oil pipe, which causes the diesel engine to extinguish automatically. Most of these failures happen on a single cylinder diesel engine with poor technical conditions. In addition, the pressure regulating spring of a single cylinder diesel injector will break down or the elasticity is too small.