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Automotive and Industrial Lubricants - Causes of High and Low Oil Pressure


Automotive and Industrial Lubricants

Engine Oil Pressure Explained





Low Oil Pressure is Not Necessarily Bad

A customer changes his brand of oil and then complains of low oil pressure. Is this perception or reality and is low oil pressure inherently bad? Why don't customers complain about high oil pressure?

In the following discussion we hope to demonstrate that:

  • Low oil pressure is not necessarily bad....in fact it can be very beneficial within reasonable limits.
  • High oil pressure is not necessarily good....in fact it can be a cause of great concern.

Back to the Basics

In the simplified schematic oil system below, oil is sucked through the pick up filter screen and up the pick up tube to the pump. This pump pushes the oil through the filter and around the engine via galleries and oil delivery holes. A pressure sending unit or transducer is usually located downstream of the filter.

Oil Pressure Schematic

Circulation of the oil is produced by a pump and not a compressor. The most important criterion for good lubrication is oil flow and not oil pressure. The oil pressure will vary considerably around the engine, declining with distance downstream from the pump. Oil pressure is negative on the suction side of the pump.


What Causes Oil Pressure?

Oil pressure is caused by the resistance of the oil to flow (viscosity) under the pumping action of the oil pump. With wide oil galleries and low viscosity oil, flow would be rapid and oil pressure low - a very desirable condition for minimizing wear. Conversely, under the same pumping conditions; with narrow oil galleries (by design or by blockage) and high viscosity oil, the oil flow will be slow, oil pressure will be high, hence resulting in less efficient lubrication.

A "good" oil will be one that has viscosity characteristics sufficient to give good hydrodynamic lubrication of the loaded surfaces, yet flows around the engine well to provide a continuous supply of fresh lubricant.

In an extreme case, a very viscous oil would never get to the critical parts to do its lubrication job. Note in this case, that engines do have by-pass valves to prevent excessive oil pressure in case of filter blockage.


Oil Pressure Gauge Interpretation


Cold Start

When and engine starts, all the oil is in the sump and the oil pressure is zero. The pump cannot begin to deliver oil or generate oil pressure until it has sucked up cold oil through the filter screen and the pick up tube. Hence, cold start lubrication is greatly assisted by having a short, wide pick up tube and oil that has very good cold flow properties.

When the oil reaches the pump it is then forced through the filter and then through the engine. However, as the oil is still cold and the oil channels are very narrow, flow is slow and a large back pressure develops so that the pressure Gauge registers a dramatic pressure increase.

As the oil circulates and warms up, it flows faster and the oil pressure declines to a stable level. It is only at this point that the engine is being properly lubricated. Until stable oil pressure is obtained wear rates are high due to inadequate oil delivery to the wearing surfaces. Under cold start conditions, a "good" oil is one which gives stable oil pressure the fastest.


Normal Operation

Under ideal circumstances, oil pressure should be stable and any large increase or decrease should be investigated for possible mechanical damage.


Causes of Low Oil Pressure
Causes of Low Oil Pressure Reading Consequences Action
Low oil level Possible catastrophic engine failure Top up oil level and investigate possibility of an oil leak
Oil not flowing into the pump at start-up Possible catastrophic engine failure Shut down the engine. Change to oil with better low temperature properties. Improve cold start-up procedures
"Lugging". Pump runs too slow to deliver sufficient oil Reduces engine life Downshift to increase rpm. Check the pump
Oil too hot, viscosity too low. Oil breakdown/engine problems; wear, deposits Check temperature Gauge and engine temperature controls. Check oil viscosity for correct grade
Worn oil pump Engine problems Replace pump
Worn bearings Engine problems Oil flows through increased clearance more easily. Replace bearings
Fuel in oil reduces viscosity Higher oil consumption. Engine wear. Avoid excessive idling. Check injectors. Change oil.
Oil change Old oil had an higher viscosity due to soot load and oxidation. New oil flows better None


Causes of High Oil Pressure
Causes of High Pressure Reading Consequence Action
Pressure remains high after cold start. Oil flows adequately into the pump, but does not flow through the oil galleries Possible catastrophic failure Shut down engine. Use oil with better low temperature properties (5W-30 or 0W-30)
Oil is thickened by soot load Potential engine failure Change oil and filter. Check injector performance. Avoid excessive idling.
Oil is thickened by oxidation Potential engine failure Change oil and filter
Oil viscosity is too high. Oil flow is poor. Potential engine failure. Consult owners manual/oil supplier for correct viscosity grade. Change oil.
Filter blocked. By-pass valve allows unfiltered oil to circulate. Reduces engine life. Change filter and oil. Investigate cause of blockage.
Deposits in oil gallery system cause increased back pressure. Reduces engine life. Change oil and filter. Use higher quality oil.
Oil too cold. Potential engine failure. Check engine thermostat system. Check to ensure correct oil viscosity.


SUMMARY

The customer/operator should be just as concerned with high oil pressure as with low oil pressure. In fact any deviations, low or high, from "normal oil pressure" should be investigated.

Higher oil pressure means that more work has to be done to pump the oil around the engine and this loss of efficiency should be minimized. High oil pressure does not equate to good oil flow and in many cases it is an indication of the opposite.

Alteratively, lower oil pressure can mean that the oil is flowing rapidly around the engine which is a very desirable condition for minimizing wear.