Low lubricating oil pressure (or no oil pressure) can be caused by several lubrication system related malfunctions. To begin the investigation, determine the engine operating conditions when the low pressure was first observed; i.e. following a service interval, at idle only, while operating on a steep grade, etc.
Lubricating Oil Level
High oil level can cause low oil pressure. If the oil level is high enough for the connecting rods to dip into during
operation, the oil can become aerated resulting in low oil pressure.
Low lubricating oil level will not normally appear as low oil pressure. Typically it will appear as an intermittent loss of
oil pressure when rounding a corner or operating on a steep grade. This condition exists when the oil level is
extremely low and the suction tube cannot pick up oil during all modes of operation.
Lubricating Oil Filter
A plugged filter will cause a gradual loss of oil pressure by approximately 69 kPa [10 psi]. The pressure will return
to normal w h e n the filter bypass valve opens. If not corrected this will result in severe engine wear, as the engine
is running on unfiltered oil when the bypass valve is open.
Lubricating Oil Gauge
Check the lubricating oil gauge and sending unit to make sure they are operating correctly by verifying the pressure
with a manual gauge.
Lubricating Oil Suction Tube
A loose suction tube, damaged gasket or crack in the suction tube can cause a temporary loss of prime for the
oil pump. The engine will have low pressure or no oil pressure at starting followed by normal oil pressure.
Bearings and Lubricating Oil Pump
A steady decrease in oil pressure over a long period of time can be an indication of worn bearings or excessive
oil pump wear.
Incorrect Lubricating Oil Pump
The capacity of the six cylinder oil pump is greater than the four cylinder. If low or high pressure occurs after
changing the pump, verify that the correct pump was used. Refer to oil pump replacement, page 2-24.
A – Four Cylinder
• 12.947 mm [0.516 inch]
B – Six Cylinder
• 17.947 mm [0.715 inch]
NOTE: Earlier model B-Series engines used lower capacity pumps. Check to be sure the correct model pump is used.
