Low or irradic oil pressure can be due to a number of things. Because of this occuring more at idle I would recommend considering the following possible problems.
1. Worn camshaft or main bearings will cause low and sometimes irradic oil pressure as pressure is lost due to excessive clearence of the bearings
2. Oil pick up tube may have become loose. If the pick up tube becomes loose you can see lost pressure or irradic readings
3. A faulty oil sending unit can definately cause irradic readings of oil pressure
4. Oil pressure relief valve may be sticking causing irradic readings due to pressure not bleeding off correctly.
Does your vehicle ever go high on oil pressure? If so how high of a reading are you getting and describe exactly the range the gauge is reading when this happens. Does it spike and come down? Does it stay at or below normal pressure but is just irradic?
Thursday, March 5th, 2009 AT 11:45 AM