During the use of diesel vehicles, there are sometimes failures such as normal operation of the starter but difficulty in starting, or unstable speed after starting, stalling, etc. Most of these failures are caused by the air entering the low pressure or high pressure oil circuit, which leads to bubbles in the fuel and discontinuous oil supply, resulting in difficulties in starting, unstable speed, stalling, etc. At this time, it is necessary to remove the air in the fuel oil circuit. High pressure and low pressure oil circuits may enter air, and low pressure oil circuit is more common. When exhausting, the air in the low-pressure oil circuit is generally discharged first, and then the air in the high-pressure oil circuit is discharged. The engine can be started normally after the air is removed, and the fault is eliminated.

Exhaust time:
1. When air enters the oil circuit due to loosening of vent screw or oil pipe connector;
2. After the fuel tank is used up and refueled again;
3. After replacing high and low pressure fuel pipes, fuel filters, fuel transfer pumps (low pressure fuel pumps), fuel injection pumps (high pressure fuel pumps) and other parts.

Low pressure oil circuit exhaust:
Low pressure oil circuit refers to the oil circuit from the oil tank to the low pressure oil passage of the fuel injection pump

The exhaust method of low-pressure oil circuit is as follows:
（1） Preparations
1. Check whether the oil tank switch is open, and check the oil volume of the oil tank. If the oil volume is too small, add diesel oil first.
2. Check whether the oil pipe joints between the oil tank and the oil transfer pump are loose, and tighten them if they are loose.
（2） Exhaust air from diesel filter
1. Loosen the vent screw on the diesel filter
2. Press the pump handle on the fuel transfer pump
3. Observe the air bleed screw that has been loosened. The discharged fuel contains bubbles. Keep pressing the pump handle on the fuel transfer pump until the fuel does not contain bubbles. Then tighten the air bleed screw on the diesel filter.
（3） Bleed the air from the low pressure passage of the fuel injection pump
1. Turn the ignition switch to ON, and the fuel cut-off solenoid valve is open.
2. Loosen the bleed screw on the fuel injection pump
3. Press the oil pump handle on the fuel transfer pump, observe the loosened vent screw on the fuel injection pump, and the discharged fuel has bubbles. At this time, continue to press the oil pump handle until the fuel does not contain bubbles, stop pumping oil, and then tighten the vent screw on the fuel injection pump.
（4） Supplementary instructions for exhaust of low pressure oil circuit:
1. The structure of diesel oil delivery pump is different, and the type of oil delivery pump is also different. For example, Jiefang and Steyr diesel engines use plunger type fuel transfer pumps. In addition to the cylindrical shape shown in Figure 3, the fuel transfer pumps also have a circular handle shown in Figure 5a. Some Dongfeng Cummins diesel engines use diaphragm type fuel transfer pumps, which are driven by hand rocker arms,
2. If there is still no oil emerging from the vent screw of the pump oil handle on the fuel transfer pump after being pressed for a long time, the possible causes and solutions are as follows:
(1) Low pressure oil circuit is blocked. First, check whether the oil tank switch is open and whether the oil volume in the oil tank is sufficient. Then remove the oil outlet pipe connector of the oil transfer pump, pump oil with the handle, and observe whether the oil outlet pipe connector of the oil transfer pump is oil outlet. If the oil outlet pipe connector of the oil transfer pump is not oil outlet, it indicates that the oil path between the oil transfer pump and the oil tank is blocked. You can check the oil from the oil inlet of the oil transfer pump to the oil tank section by section to solve the problem of blocked oil path.
(2) For Dongfeng Cummins diesel engine using diaphragm type oil transfer pump, if the eccentric wheel driving the rocker arm in the oil transfer pump is at the highest jacking position, the diaphragm cannot move up and down when the rocker arm is moved, and the hand rocker arm cannot pump oil, then the crankshaft should be slightly rotated so that the eccentric wheel is at the position where the rocker arm in the oil transfer pump is not jacked, which can solve the problem that the hand rocker arm cannot pump oil.

High pressure oil circuit exhaust:
High pressure oil circuit refers to the oil circuit from the high-pressure oil pipe connector of the fuel injection pump to the fuel injector.

The high-pressure oil circuit is vented as follows:
1. Loosen 2-3 fuel injector high-pressure fuel pipe connectors.
2. Turn the ignition switch to ON, press the auxiliary starting switch of the engine, and let the starter drive the crankshaft to rotate. At this time, it can be seen that diesel fuel with bubbles is ejected from the high-pressure fuel pipe connector of the loosened fuel injector. Stop rotating until the high-pressure diesel fuel is free of bubbles, and then tighten the high-pressure fuel pipe connector of the fuel injector.
Pay attention to the exhaust of the high-pressure oil circuit. Start the engine with the auxiliary starting switch. The duration of each time should not exceed 30s and the interval of each time should be at least 2min to avoid damaging the battery and starter.
Summary: Diesel vehicle oil way exhaust is a necessary skill for drivers and repairmen, which will inevitably be encountered in vehicle use and maintenance. When implementing the oil way exhaust operation, sometimes it is smooth and the exhaust operation is easy, but sometimes it is difficult to press the oil handle of the oil delivery pump for a long time and no oil emerges at the air release screw. Sometimes it is difficult to do nothing for half a day or even a day, which is where the difficulty lies. The first trick to solve the problem is to check the low pressure oil circuit block by block. The second is that for engines using diaphragm type oil transfer pumps, the eccentric wheel is in an improper position, which is often the key to causing no oil to emerge when pressing the oil handle of the oil transfer pump for a long time. If you do not understand this, you will definitely be unable to solve the problem after a long time of troubleshooting. As long as you master these two points, you can solve the problem easily.