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1. Phenomenon
In the normal operation of the engine, the indicator value of the oil pressure gauge pointer is lower than the requirements of the technical documents.
2. The reasons are as follows
According to the composition and working principle of the lubrication system, the oil pump sucks the oil from the oil pan and raises the pressure. After being filtered, the oil is pumped to the friction surface of the part and then flows back to the oil bottom from the fitting gap of the part. Lubrication system pressure depends on the pump oil efficiency of the pump and the flow resistance of the oil in the lubrication system. If the oil pump efficiency of the oil pump decreases or the flow resistance of the lubrication system decreases, the oil pressure will decrease. It can be seen from the lubrication oil circuit that the flow resistance of the lubrication oil circulation circuit is equal to the sum of the reciprocal oil flow resistance of the parallel branch oil. Pressure lubrication parts of camshaft journals, connecting rod journals, crankshaft journals, rocker shafts, etc., if these lubrication parts with the clearance is too large, or the lubrication system has abnormal leakage and pressure regulating valve adjustment pressure is too low, etc., Both will reduce the flow resistance of the lubrication system and reduce the oil pressure.
The common causes of oil pump pump efficiency drop and lubrication system oil flow resistance decrease are:
(1) Wear of the oil pump The gear of the oil pump inevitably wears out. If the oil contains mechanical impurities, it will accelerate the wear process. When worn, the internal leakage increases, so the pumping efficiency decreases accordingly.
(2) Reduced oil intake pump
The oil filter is used to filter large mechanical impurities in the oil. The mechanical impurities adhering to the filter will increase with the prolonged use of time, resulting in a small cross-section of the oil-absorbing passage and a decrease in oil ingested by the oil pump, resulting in a drop in the oil pressure of the lubrication system and even no pressure.
In the oil suction section of the oil pump, if there is a serious shortage of oil in the oil pipe or joint or the oil at the bottom of the oil pump, the degree of vacuum in the oil suction chamber of the oil pump decreases, so that the oil pump does not fully absorb oil, resulting in a low oil pressure in the lubrication system.
(3) Large amount of leakage
The basic principle that the oil pump can generate pressure is that the oil flows in the oil passage and there is resistance. If the oil passage of the lubrication system leaks, the pressure limiting valve sets the pressure too low or closes, the crankshaft or the cam journal is worn due to wear and clearance. If it is too large, the leakage amount of the lubrication system will increase, and the oil pressure in the system will decrease accordingly with the increase of the leakage amount.
(4) Plugging of oil filter or cooler
The function of the oil filter is to further filter small mechanical impurities. When used for too long, the filtered mechanical impurities collect on the filter element. With the prolonged use of time, the mechanical impurity content on the outer surface area of ​​the filter element increases, blocking the flow passage of the lubricating oil, resulting in a decrease in the oil pressure at the lubrication portion.
Oil impurities on the inner wall of the oil cooler of the oil cooler adhere to mechanical impurities or gums, which not only results in poor heat dissipation of the oil, but also reduces the flow cross-section of the oil. In severe cases, jamming may occur, resulting in reduced oil pressure in the lubrication section.
(5) Effect of Engine Oil Viscosity
The oil viscosity actually refers to the magnitude of the internal frictional resistance when the oil flows. The internal frictional resistance of the oil flow is small and its fluidity is good. On the contrary, when the internal frictional resistance of the oil flow is large, its fluidity is poor, so the viscosity is the most important measure of oil.
The oil viscosity changes as the temperature of the random oil changes. The viscosity is large when the oil temperature is low, and the viscosity is small when the temperature is high.
When the viscosity of the oil is reduced due to excessive temperature or dilution of other substances, the leakage amount of the lubrication system increases and the pressure decreases. Conversely, when the viscosity of the oil is large, the fluidity is poor and the oil circuit is blocked, which also reduces the oil pressure in the friction area.
(6) Improper adjustment of pressure limiting valve
According to the working principle of the pressure limiting valve, the pressure limiting valve is based on balanced springs and ball valves (or poppet valves) to limit the oil pressure, so that the oil pressure does not exceed the specified value of the technical file. When the oil pressure exceeds the specified value, it overcomes the elastic force of the spring to push the valve open to release the pressure in the system. When the oil pressure is lower than the spring force, the valve closes under the action of the spring. From this point of view, the oil pressure of the lubrication system depends on the size of the spring force. If the adjusted spring force is too small or the spring is fatigued and the spring force is too small, the oil pressure in the system will be reduced.
In addition, the pressure-limiting valve is not closed due to the influence of mechanical impurities, and it also causes the oil pressure to drop.
(7) Influence of oil pressure display device
The oil pressure display device includes an oil pressure gauge and an oil pressure sensor, which are used to reflect the oil pressure of the engine lubrication system. If the oil pressure gauge or oil pressure precipitator fails, the reflected pressure value will be distorted, causing it to be mistaken for a malfunction in the lubrication system.
3. Diagnosis and elimination
(1) Check the oil pressure display device
Use replacement method to check, that is to replace the good oil pressure gauge or sensor. If the oil pressure after the replacement is normal, the oil pressure display device is faulty. If the oil pressure display device is normal, the oil pressure is reduced, and the fault is in the lubrication part, judge according to the following method.
Judging from the oil temperature
If the oil temperature is too low, the oil pressure is low, and when the temperature rises, the oil pressure is normal, indicating that the oil is too thick and the oil passage is not smooth. If the original oil filter is clogged with impurities, the oil pressure will be reduced, but the oil pressure will be reduced. However, the oil pressure is still within the specified range. When the oil viscosity is too high, the oil filter will be further blocked, so that the oil pressure will drop again and be low. At the specified value, the oil pressure is too low; some diesel engine oil pressure display device sensors are located at the end of the lubricating oil path. When the filter is difficult, the oil pressure is reduced. When it is lower than the specified value, the oil pressure is too low. Then, we should further investigate the reasons for the thickened oil (the choice of engine oil number and season, oil deterioration, etc.) and the obstruction of the oil channel (such as filter section, filter screen, air-cooled radiator, etc.). After identification, it should be excluded.
If the oil pressure drops when the oil temperature is high, the oil viscosity is too small, or there is a large gap or friction between the friction parts. Should identify the reasons for the smaller viscosity (such as improper selection of engine oil grades, poor oil quality, poor viscosity and temperature stability, and the possibility of mixing diesel oil with engine oil and diluting engine oil), observe whether there is oil leakage, or the engine Longer and pressure-lubricated matching parts (such as the crankshaft and the bearing, the connecting rod shaft diameter and the bearing) have a large gap and should be further identified and targeted.
(2) Judging by the engine's service life
If the oil pressure is too low, the failure occurs during the run-in period. Most doubts are that the metal shavings in the machine part or the molding sand and other acrobatics block the filter, or that the pipe bends through the oil, or it is improperly assembled and leaks oil. The place (such as the bearing fit clearance is too large, the pipe joints or joints at the joint plane is not sealed). It should be easy to find out the reason and eliminate it afterwards.
(3) Diagnose according to the abruptness and gradual change of oil pressure drop
If the oil pressure suddenly drops, indicating that the pipe or gasket is broken, the leak should be identified and removed.
If the oil pressure is gradually reduced with the use of time, the oil pressure should be gradually reduced, check for clogging, wear, engine oil deterioration, etc., find out the cause and eliminate it.
If the engine oil pressure is gradually reduced during normal use, it means that there is a shortage of oil at the bottom of the oil, blockage of the oil line, increase of the fitting clearance of the pressure lubricated parts, deterioration of the oil, etc., which are caused by simultaneous reduction of oil pressure caused by multiple causes. In general, a single cause, although having an impact on oil pressure, will not be too low. During the diagnosis, first check the oil storage and deterioration, if both of these are not in compliance with the requirements, it is one of the reasons for the reduction of oil pressure. The filter should be removed to check the clogging of mechanical impurities. If there is obvious blockage, It is the fault. The clearance between the crankshaft and the associated bearing can also be checked. If the wear increases, it is also one of the causes of the oil pressure drop.
(4) Check the pressure limiting valve
Remove the lower limit pressure valve after cleaning and refit (adjust the spring force). If the oil pressure increases, the oil pressure is too low.
After the above examination, analyze the causes and eliminate the symptoms.
If the engine is used near the overhaul period (depletion period), the oil pressure is too low, mostly due to the increased leakage of the lubrication system, engine oil deterioration, and plugging of the filter or piping, sometimes accompanied by connecting rods, etc. The knocking sound should be overhauled.
Hubei Cheng Li Special Purpose Vehicle Co., Ltd.
Low oil pressure