The reason is that the piston inside the pump body of the fuel injection pump (i.e. the high pressure oil pump) has been worn out for a long time and the pressure does not cannot be reached, it will therefore stall. When the car is idling at high speed, the engine speed is fast, so there will be no fire extinguishing. When the idle speed is low, the engine speed is slow, the piston pressure in the fuel injection pump will not reach the injection pressure and the fuel injector will not atomize the diesel, it will stall SO.
There are 6 sets of injectors on the engine cylinder head. Factory pressure is usually calibrated at around 220 pressure because it is a new oil pump and piston. Pump oil pressure is high. However, if the engine runs for a long time, the piston pressure will gradually decrease, while the pressure of the assembly injet will always remain at around 220 pressure. At this point you need to adjust the injector pressure or replace it. Regarding the fuel injection pump piston, it is recommended to take the high pressure fuel pump and fuel injection nozzle assembly to a professional fuel pump calibration expert for calibration.