What is the cause of the lowest oil pressure

2022-08-07
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What is the cause of low oil pressure

1. Phenomenon

under the normal operation of the engine, the indicator value of the oil pressure gauge is lower than the requirements of the technical documents

2. Cause analysis

it is known from the composition and working principle of the lubrication system that the oil pump sucks oil from the oil pan and increases the pressure. After filtering, it is pressed to the friction surface of the parts, and then flows back to the oil pan from the fitting clearance of the parts. The pressure of the lubrication system depends on the pump oil efficiency of the oil pump and the flow resistance of the oil in the lubrication system. If the pump oil efficiency of the oil pump decreases or the flow resistance of the lubrication system decreases, the oil pressure will decrease. It can also be seen from the lubricating oil circuit that the flow resistance of the oil circulation circuit of the lubrication system is equal to the reciprocal sum of the oil flow resistance of the parallel branch. Camshaft journal, connecting rod journal, crankshaft journal, rocker arm shaft, etc. at the pressure lubrication parts, if the fitting clearance of these lubrication parts is too large, or the lubrication system has abnormal leakage and the pressure regulating valve pressure is too low, the flow resistance of the lubrication system oil circuit will be reduced, and the oil pressure of the machine will be reduced

the common reasons for the decrease of pump oil efficiency of the oil pump and the decrease of oil flow resistance of the lubrication system are:

(1) the oil pump is worn. The gear of the oil pump must be worn when it is working. If the oil contains mechanical impurities, the wear process will be accelerated. When worn, its internal leakage increases, so the pump oil efficiency decreases accordingly

(2) the oil volume of the suction oil pump is reduced

the oil strainer is used to filter large mechanical impurities in the oil. The machine adhered to the filter collector also sends a clear signal to us that we need to find more reliable and environmentally friendly materials to replace the mechanical impurities, which will increase with the extension of service time, resulting in a small cross-section of the oil suction channel, a reduction in the oil suction by the oil pump, and a drop in the oil pressure of the lubrication system, or even no pressure

in the oil suction section of the oil pump, if there is air leakage at the oil pipe or joint or there is a serious shortage of oil at the oil bottom, the vacuum in the oil suction chamber of the oil pump will drop, so that the oil suction of the oil pump is not full, resulting in low oil pressure in the lubrication system

(3) large leakage

the basic principle that the oil pump can generate pressure is that the oil flow in the oil passage has resistance. If the oil passage of the lubrication system has leakage, the set pressure of the pressure limiting valve is too low or not closed tightly, and the crankshaft or camshaft journal has too large wear fit clearance, the leakage of the lubrication system will increase, and the oil pressure in the system will decrease accordingly with the increase of the leakage

(4) the oil filter or cooler is blocked

the role of the oil filter is to further filter small mechanical impurities through 4. After long use, the filtered mechanical impurities are collected on the filter element. With the extension of service time, the mechanical impurities in the outer surface area of the filter element increase, blocking the lubricating oil flow channel, resulting in the reduction of oil pressure at the lubricating part

there are mechanical impurities or colloids adhered to the inner wall of the oil pipe of the oil cooler, which will not only cause poor heat dissipation of the oil, but also reduce the flow section of the oil. In serious cases, there will be blockage, resulting in reduced oil pressure at the lubrication part

(5) influence of oil viscosity

oil viscosity actually refers to the internal friction resistance when the oil flows. When the oil flows, the internal friction resistance is small, and its fluidity is good. On the contrary, when the internal friction resistance of oil flow is large, its fluidity is poor, so viscosity is the most important measure of oil

the oil viscosity changes with the oil temperature. The viscosity of the engine oil is high at low temperature and low at high temperature

when the viscosity of engine oil decreases due to high temperature or dilution of other substances, the leakage of lubricating system increases and the pressure decreases. On the contrary, when the oil viscosity is high, the fluidity is poor and the oil circuit is blocked, which will also reduce the oil pressure at the friction part

(6) improper adjustment of the pressure limiting valve

it can be seen from the working principle of the pressure limiting valve that the oil pressure of the pressure limiting valve is limited by the balance spring and ball valve (or cone valve), so that the oil pressure does not exceed the specified value in the technical documents. When the oil pressure exceeds the specified value, the valve will be pushed open to relieve the pressure in the system against the spring force; When the oil pressure is lower than the spring force, the valve closes under the action of the spring. Therefore, the oil pressure of the lubrication system depends on the spring force. If the adjusted spring force is too small or the spring force is too small due to fatigue, the oil pressure in the system will be reduced

in addition, the pressure limiting valve is not tightly closed due to the influence of mechanical impurities, which will also reduce the oil pressure

(7) influence of oil pressure display device

oil pressure display device includes oil pressure gauge and oil pressure sensor, which is used to reflect the oil pressure of engine lubrication system. If the engine oil pressure gauge or the engine oil pressure transmitter fails, the pressure value reflected will be distorted, making it mistakenly believe that the lubrication system has failed

3. Diagnosis and troubleshooting

(1) check the oil pressure display device

use the replacement method, that is, replace the oil pressure gauge or sensor respectively. If the oil pressure display is normal after replacement, it indicates that the oil pressure display device is faulty. If the oil pressure display device is normal, the oil pressure decreases, and the fault is in the lubrication part, the following methods shall be used for judgment

judging from the oil temperature

if the oil temperature is too low, the oil pressure is low, and the oil pressure is normal after the temperature rises, it means that the oil is too thick and the oil passage is not smooth. If the original impurities of the oil filter are blocked, although the oil inlet volume is affected to a certain extent, the oil pressure is also reduced, but it 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 drops again and is lower than the specified value, then the oil pressure is too low; The sensor of some diesel engine oil pressure display devices is set at the end of the lubricating oil circuit. When it is difficult to pass through the filter, the oil pressure decreases. When it is lower than the specified value, the oil pressure is too low. Then, it is necessary to further find out the reasons for the excessive oil thickness (whether the oil brand is suitable for the season, whether the oil is deteriorated, etc.) and the oil duct blockage (such as filter joint, filter, air-cooled radiator, etc.). After investigation, it should be eliminated

if the oil pressure drops when the oil temperature is high, it indicates that the oil viscosity is too small, or the fitting clearance of friction parts is large or there is leakage. Further, find out the reasons for the viscosity reduction (for example, improper oil brand selection, poor oil quality, poor viscosity temperature stability, or it may be that the oil is diluted due to the mixing of diesel and engine oil), and observe whether there is oil leakage, or the clearance between pressure lubricated mating parts of the engine (such as crankshaft and bearing, connecting rod shaft diameter and bearing) that have been used for a long time is too large. Further, find out and deal with it accordingly

(2) judge according to the service life of the engine

if the oil pressure is too low, the fault occurs in the running in period. Most of them suspect that the filter is blocked by acrobatics such as metal scraps in the machine parts or molding sand during casting, or the pipeline is bent and the oil is not smooth, or there is oil leakage due to improper assembly (such as excessive bearing fit clearance, poor sealing at the pipe joint or joint plane). It should be easy before difficult, and then find out the cause and eliminate it

(3) diagnose according to the sudden and gradual decrease of oil pressure

if the oil pressure decreases suddenly, it indicates that the pipe or gasket is broken, and the oil leakage shall be found out and eliminated

if the oil pressure decreases gradually with the extension of service time, check whether the oil is blocked, worn, deteriorated, etc., find out the causes and eliminate them

if the engine oil pressure decreases gradually during the normal service period, it indicates that there are many reasons for the decrease of engine oil pressure, such as the shortage of engine oil at the oil bottom, the blockage of oil circuit, the increase of fitting clearance at the pressure lubrication part, the deterioration of engine oil, etc. Generally, although a single reason has an impact on the oil pressure, it will not be too low. During diagnosis, first check the oil storage and deterioration. If these two items do not meet the requirements, it is one of the reasons for the reduction of oil pressure. Then remove the filter to check the blockage of mechanical impurities. If there is obvious blockage, it is the fault. You can also check the fit clearance between the crankshaft and the bearings of related parts. If the wear increases, it is also one of the reasons for the reduction of oil pressure

(4) check the pressure limiting valve

remove the pressure limiting valve, clean it, reassemble it and adjust it (increase the spring force). If the oil pressure increases, it is the fault of low oil pressure

after passing the above examinations, analyze the causes and eliminate the symptoms

if the engine is nearing the overhaul period (wear and tear period) and the oil pressure is too low, most of them are caused by the increased leakage of the lubrication system, the deterioration of the oil, the blockage of the filter or pipeline, and sometimes accompanied by the knocking sound of the connecting rod, etc., overhaul should be carried out

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