Diagnosis of low-pressure fuel line of diesel engine with gas seepage

The fuel line of a diesel engine can be divided into a negative pressure line, a low pressure line and a high pressure line.

The negative pressure line is the line from the fuel tank to the fuel pump. The pressure in this section of the fuel circuit is lower than atmospheric pressure when the engine is operating, so the fault is only likely to be an air leak, not an oil leak. The air leak will reduce the vacuum in this section of the oil circuit, so that the fuel (diesel) entering the fuel pump is reduced, resulting in insufficient fuel supply to the fuel pump or even a loss of fuel, and the engine cannot be started.

When the amount of air leakage is small, the fuel delivery pump can still continue to supply fuel, but the engine is more difficult to start or the engine starts soon after the self-stop. The reason for the above phenomenon is: air is compressible, a small amount of air into the oil circuit from the injection pump plunger oil hole is sucked into the pump cavity after becoming a bubble, its volume changes with the reciprocating movement of the injection pump plunger.

When the plunger in the up (pressure oil) stroke, the bubble is compressed, so that the oil pressure can not rise to the required injection pressure; when the plunger in the down (return) stroke, the bubble and expansion and return to the original volume, so that the pump cavity can not produce a vacuum of oil, as is blocked in general. Therefore, there should never be even the slightest air leakage point in the negative pressure oil circuit.

The fuel of a diesel engine is sucked out of the fuel tank by the delivery pump and delivered to the injection pump. If the oil circuit is seeping with air, the delivery pump is unable to deliver fuel. Air is elastic and compressible.

When there is air in the oil circuit of the diesel engine fuel supply system, with the reciprocating movement of the injection pump plunger, the bubble produces a change in volume, when the plunger in the compression stroke, the bubble is compressed, so that the oil pressure can not rise to the specified injection pressure. When the plunger in the suction stroke, the bubble and expansion to restore the original volume, so that the oil circuit can not produce the suction into the oil, so there is air like a blockage of insufficient oil supply, or even interrupt. The engine gradually stalls while driving; it does not start again after stalling; open the high-pressure oil pump bleed screw, pump oil with a hand oil pump to exclude the air in the oil circuit, and then start the engine can fire, but after a few moments the phenomenon of stalling occurs again, you can determine that the oil circuit into the air.

When the diesel engine is working, the pressure of the fuel tank to the pump section of the oil circuit is lower than the atmospheric pressure, so as long as there is a small air seepage point will be external air intake, thus causing the engine to work unstable or even stalled. Confirmation of this section of the oil circuit whether air seepage method, is when the engine is off, loosen the injection pump bleeder screw, with the hand oil pump pump, if found at the bleeder screw began to discharge a large number of air bubbles, and repeatedly with the hand oil pump, still can not make the oil flow completely free of air bubbles, it can be determined that the section of the oil circuit there is air seepage point. In addition to the oil level in the tank is low to close to the suction port and the air into the oil pipe, this section of the oil circuit gas seepage point more in the oil delivery pump into the oil port oil pipe joints, generally for the copper gasket damage or improper installation, oil pipe cracks and other very easy to seepage here.

Diesel engine work, from the oil pump to the injection pump section of the oil pressure is higher than the atmospheric pressure, even if the oil circuit has seepage point, can only leak oil, can not seepage, so will not affect the normal operation of the engine, but when the engine is off, due to the oil from the leakage point after seepage, so that the oil pressure higher than the atmospheric pressure can not be maintained, and even cause air backflow, resulting in the difficulty of starting again. In order to confirm whether the section is seepage, available a fuel pipe from the oil pump jump over the diesel filter directly to the injection pump, and then start the engine after the exhaust for observation, if the fault disappears, you can determine the existence of the section of the oil circuit seepage point, if the fault still exists, it may be the injection pump plunger sleeve positioning screw, bleed screw, etc. caused by oil seepage.

Low-pressure oil circuit air seepage fault elimination methods are as follows.

When encountering low-pressure oil leakage, first check whether the diesel filter, injection pump, etc. bleed screws are tightened; then check the various pipeline joints, especially the hose part is prone to air seepage phenomenon, should be carefully examined to identify the causes, if necessary, to be eliminated. Open the air release screw and use the hand oil pump to remove the air in the oil circuit; tighten the pipe joints and remove the seal that is not tight.

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 the exhaust operation must be carried out. When the oil pump pumps out all the diesel without air bubbles, you can tighten the air release bolt. Then start the engine, check whether there are any air leaks in each department, and repair them if necessary, until there is no leakage.