If the air in the oil circuit cannot be completely discharged, after running for a period of time, the exhaust pipe will make a noise and intermittently emit white smoke. The rotate speed of engine will decrease, the work will be weak, and in severe cases, it will automatically shut down. The air in the oil circuit mainly comes from two aspects.
1. Low pressure oil circuit sucks in air
The reason for the intake of air in the low-pressure oil circuit is that, first, the diesel safe fuel is exhausted, and the vent hole on the fuel tank cover is blocked, creating a vacuum inside the tank and allowing air to be drawn in. The second issue is that the sealing gasket between the hand oil pump on the fuel pump and the pump body is broken or uneven, causing air to enter. Thirdly, the piston or cup of the hand oil pump is severely worn, and air enters the low-pressure oil circuit through the gap between the piston or cup and the pump barrel from the upper part of the hand oil pump. The fourth issue is that the sealing gasket of the oil pipe jointing the low-pressure pipeline is uneven, damaged, or the hollow screw is not tightened, causing air to enter. Fifth, the low-pressure fuel pipe of the fuel supply system is ruptured, causing air to enter. The inspection method is to remove the low-pressure oil pipe, wipe the joint clean, and block one end of the oil pipe with fingers, while using a nozzle to extract air from the other end. If the oil pipe is not broken, a vacuum has basically formed inside the oil pipe and the tongue tip is sucked in; If you always can't suck the tip of your tongue, it means the oil pipe is broken. Sixth, mechanical impurities cause the return valve to not close tightly, and diesel is discharged through the return valve, resulting in excessive pressure in the fuel injection pump passage, less fuel injection volume, and incomplete air in the oil circuit.
2. High pressure gas is flushed back from the fuel injector into the fuel supply circuit
At this time, if the needle valve of the fuel injector is stuck in the opening position, the sealing of the fuel outlet valve of the same cylinder is not tight, the needle valve and the guide part of the needle valve body are severely worn, the high-pressure spring inside the fuel injector is broken, and the fitting hole between the injector plunger and the needle valve is worn, then after each cycle of cylinder operation, high-pressure air will flow back into the fuel supply circuit.
The reason why high-pressure air flows into the fuel supply system is that after the compression stroke of the diesel engine, at a certain angle before the top dead center of the piston, the injector injects diesel into the cylinder. At the moment when the fuel injector is turned on, the gas in the cylinder forms a passage with the oil circuit (the gas in the passage has a pressure difference that varies with time). When the quality of the fuel injector is poor, the high-pressure gas in the cylinder will flow back from the injection hole through the sealing cone surface, through the gap at the needle valve of the fuel injector, the tappet hole in the fuel injector body, and the return pipe of the fuel injector into the diesel filter, until it enters the oil circuit. Over time, this will result in the fault of diesel supply being blocked by air.
For diesel engines with a return pipe that is not directly connected to the diesel filter (taking the NO.4115 diesel top as an example), high-pressure air may enter the fuel injection pump cover through the inlet hole of the injector body, the high-pressure oil pipe, the gap between the outlet valve and the seat, and return to the fuel transfer pump through the return valve hole on the pump cover.
The fuel supply system of a diesel engine is roughly divided into several sections, including the fuel tank, oil-water separator, fuel transfer pump, fuel fine filter, and high-pressure fuel pump. We need to operate them in sections during exhaust. The specific process of oil circuit exhaust operation is as follows:
1. Loosen the screw of the oil outlet pipe joint of the oil-water separator, then press the hand oil pump on the oil-water separator until there are no bubbles in the diesel oil flowing out of the oil outlet pipe, and then tighten this screw;
2. Loosen the screw of the inlet pipe joint of the fuel pump, then press the hand oil pump on the oil-water separator until there are no bubbles in the diesel oil flowing out of this pipe joint, and then tighten this screw;
3. Loosen the screw of the outlet pipe joint of the fuel transfer pump, then press the hand oil pump on the fuel transfer pump until there are no bubbles in the diesel oil flowing out of this pipe joint, and then tighten this screw;
During this process, it should be noted that sometimes pressing the hand pump does not respond and feels like it is empty. This situation may be due to the oil pump diaphragm being in the pumping position and the manual oil pump not functioning. At this point, all you need to do is turn the engine once.
4. Loosen the screw or vent screw of the fuel fine filter outlet joint, and then press the hand oil pump on the fuel transfer pump until there are no bubbles in the diesel oil flowing out of this pipe joint, and then tighten this screw; Due to the large size of the fuel fine filter, this process takes a long time;
5. Loosen the exhaust (return) screw on the high-pressure oil pump, continue to press the hand oil pump on the fuel transfer pump until there are no bubbles in the diesel oil flowing out from the exhaust screw, and then tighten the exhaust screw; Continue pressing the manual oil pump on the fuel transfer pump until the return valve on the high-pressure oil pump makes a "squeaking" return sound.
After this operation, the air in the low-pressure oil circuit is basically eliminated. At this time, there is a lot of resistance when pressing the hand oil pump, and each press requires a lot of force. Then we continue to press the manual oil pump a few times, start the engine, and usually it can start smoothly.
The method to check for fuel injector leakage is:
Remove the fuel injector main return pipe from the diesel filter and allow it to communicate with the atmosphere. Then block the return pipe hole of the diesel filter, exhaust the air in the oil circuit, and start the diesel engine again. If there is no more gas in the fuel injection pump, it can be concluded that the fuel injector is leaking. At this point, all the return pipe joints of each injector should be removed to further determine which injector is leaking air; If there is air leakage at the return hole of a fuel injector, it can be determined that the injector has a leakage fault.
For diesel engines where the fuel injector return pipe is not connected to the diesel filter, air can be removed first, and then the high-pressure fuel pipe joints on the fuel injection pump can be loosened one by one while the diesel engine is running. If a joint continues to emit air after being loosened, it indicates that the cylinder injector is leaking air.
The troubleshooting method for low-pressure oil circuit leakage is as follows:
When encountering low-pressure oil leakage, first check whether the air release screws at the diesel filter, fuel injection pump, etc. are tightened; Recheck the joints of various pipelines in the oil circuit, especially the hose section, which is prone to air leakage. Carefully inspect and identify the cause, and if necessary, eliminate it. Open the air release screw and use a manual oil pump to remove air from the oil circuit; Tighten all pipe joints and remove any loose seals.
When replacing the filter element or cleaning the oil-water separator, a large amount of air will also enter the low-pressure oil circuit, so exhaust operation must be carried out. When the fuel pump pumps out diesel without bubbles, the bleed bolt can be tightened. Then start the engine, check if there are any leaks in any parts, and repair them if necessary until there are no leaks.