Case Analysis And Explanation

The normal engine oil working pressure of the diesel engine shall be between 0.25-0.35 MPa, which can be slightly higher in case of new engine or low engine oil temperature, and slightly lower in case of old engine or high engine oil temperature. If the engine oil pressure is lower than 0.2MPa or changes with the speed, even suddenly drops to zero during engine operation, it indicates a problem in the engine. The engine shall be shut down to find out the causes immediately, and can be restarted after the faults are eliminated. Otherwise, a serious incident of bush-burning and cylinder scoring will be caused. So, what are the causes of excessively low engine oil pressure of diesel engine?

1. Engine oil pressure drop due to insufficient oil pump capacity of engine oil pump
The engine oil pump is divided into two types: gear pump and rotor pump. Some gear pumps (e.g., engine oil pump on 4125 diesel engine) have low roughness of joint surface of pump cover and pump body. The original design is not provided with a gasket. If a paper pad is added during the inspection, the end clearance of the gear will be enlarged, and the oil pump capacity will be reduced. However, there is a gasket on the joint surface of the pump and the cylinder. If the gasket is neglected of being mounted, the pressure engine oil from the oil pump will leak outside, reducing the engine oil pressure. For some rotor pumps, the external rotor has chamfer on one side and no chamfer on the other side. If it is not installed reversely, the oil pump capacity can be reduced by 40% to 60%. In addition, when the radial and end clearance of gear or rotor is excessive due to wear, the oil pump capacity of the pump and the engine oil pressure will be reduced. For the repaired engine oil pump, the opening pressure of pressure-limiting valve shall be adjusted on the test bench to keep it to be between 0.5 and 0.7 MPa. If the pressure is too high, the engine oil pump wear will be accelerated; if the pressure is too low, the main oil channel pressure will be reduced. The engine oil shall be filled up first during engine oil pump installation to avoid oil suction failure due to air in the pump. The connection of the engine pump and oil suction plate must be sealed. The oil pump capacity will be also reduced in case of air leakage.

2. Blockage of engine oil filter causes the engine oil pressure drop
When the engine oil filter is blocked and engine oil fails to pass through smoothly, the safety valve mounted on the filter base will be opened, so the engine oil will flow into the main oil passage. However, if the safety valve fails to be timely opened due to excessive opening pressure, the internal leakage of engine oil pump will increase, the oil supply volume of main oil passage will be reduced, and the engine oil pressure will be also lowered accordingly. The engine oil filter shall be maintained on schedule, and the opening pressure of safety valve shall be correctly adjusted to 0.35-0.45MPa. In addition, if the safety valve spring is aged and valve seat and steel ball are closed non-tightly, a large amount of engine oil will also leak accordingly, and a large amount of bubbles will be generated in the engine oil of oil pan. At this point, dipstick can be pulled out, you can see the engine oil covered with bubbles.

3. The oil return valve damage causes the engine oil pressure drop
To maintain the normal engine oil pressure of main oil passage, an oil return valve is mounted here. If the oil return valve spring is softened or inappropriately adjusted, the matching surface of the valve seat and steel ball will be worn or jammed by dirt, and the valve will not be closed tightly. Its oil return volume will greatly increase and the engine oil pressure of main oil passage will also drop accordingly. The oil return valve shall be inspected to adjust the opening pressure to 0.28-0.32MPa. In addition, if the opening pressure of oil return valve is too low, the oil return valve will be frequently opened and closed during equipment operation, and the engine pressure gauge hand will swing back and forth. The main oil passing is also easy to have bush bushing incident due to insufficient oil supply.

4. Oil leakage of engine oil radiator causes engine oil pressure drop
Leaking engine oil causes the machine to be dirty and the oil pressure to drop. The radiator should be removed and repaired, and can be mounted after the pressure inspection passes. For same machines such as E200 excavator made in the US and T-120 bulldozer made in China, the engine oil radiator is directly mounted in the cooling system of engine body. If the radiator leaks oil, the engine oil pressure will be higher than the cooling water pressure, and the engine oil in radiator will flow into the water; after the machine is shut down, the cooling water will also permeate into the radiator, and the engine oil will be quickly emulsified and deteriorated. The radiator shall be taken out for repair welding, and the failed engine oil shall be replaced before it re-operates.
In addition, if the engine oil radiator and piping are blocked by dirt, the oil flow will decrease and the pressure will drop due to the increase in resistance. It should be cleared in time to restore its normal circulation capacity.

5. Excessive clearance between crankshaft and bearing bush causes engine oil pressure drop
After the machine is used for a long time, the fit clearance between the crankshaft and the connecting rod or the crankshaft and the main bearing bush will be gradually increased, the oil wedge will not be formed, and the engine oil pressure will be continuously decreased. According to the measurement, the oil pressure drops by 0.01 MPa per 0.01 mm of increased clearance. Therefore, this is one of the main causes for the low oil pressure, and can also be used as a main indicator to judge the extent of crankshaft and bearing wear and the overhaul of the diesel engine. The crankshaft should be ground and the corresponding size of the bearing bush should be selected to restore the fit clearance to the normal technical standards.

6. Fluctuated oil pressure due to blockage of oil suction plate
Generally speaking, the engine oil pressure is higher at the time of the large throttle than the one at the small throttle, but sometimes there are abnormal conditions. If the oil is too dirty and sticky, the oil suction plate will be easily blocked. Therefore, certain pressure can be still established in the main oil passage due to the limited oil suction of engine oil pump and the oil pressure is normal when the diesel engine runs at low speed; however, when the diesel engine runs at a high speed, oil suction volume of engine oil pump will be significantly reduced due to the excessive resistance of the oil suction plate. Thus, the oil supply of the main oil passage will be insufficient and the oil pressure will drop accordingly. The oil suction plate should be cleaned and replaced with new engine oil. In addition, when the oil return valve spring of the main oil passage breaks or is in clamping stagnation with the hole wall due to the bending eccentric wear, the valve opening and closing shall overcome both the spring resistance and the clamping stagnation force, so it is not opened and closed flexibly. If the diesel engine runs at the small throttle, the valve will be closed and main oil passage will still maintain certain pressure; however, when it runs at large throttle, the oil return valve will be suddenly opened due to the sharp increase of oil supply volume, and the engine oil pressure will drop accordingly. Moreover, the clamping stagnation of spring prevents the oil return valve from timely returning, and it is more difficult to maintain the required pressure in the main oil passage compared with the running at the small throttle. That’s one of causes of the engine oil pressure at the large throttle lower than the one at the small throttle.