This problem is driving me nuts, I just bought a 93 850 with 111k for my son and we are having one little problem.
Start the car in the morning and the oil light comes on. Stays on until engine speed is raised to over 1500 RPM for just a second or two then the light goes off. Drive the car for hours and never see the light come on. Let the car sit for a few hours and the problem comes back.
Trouble Shooting:
Removed the oil pressure switch and installed a gauge, start the engine cold and get about 5 to 8 psi. at idle, raise speed to 1500 RPM and the pressure climbs to about 80 psi. cold and as the engine heats up the pressure drops to about 25 psi. at idle and 80 psi. at 4000 RPM. Never drops below 20 psi no matter how hard you drive it. According to the book this appears within spec.
So I changed the oil and filter (Mann filter and Mobil 1 10w40)= same problem. Next removed a quart of oil added a quart of Gunk motor flush, ran 15 minutes drained oil, replaced with new oil and filter (Mann filter and Mobil 1 10w30)= same problem.
Next I dropped the pan and replaced the O rings on pickup tube and pan, genuine Volvo parts including the hot pink gasket liquid. The pan was really clean for 115k no sludge just the usual varnish and the tube was clear and clean, but I still scrubbed them out with solvent. New filter and oil again and for the first four startups cold, normal oil pressure 70 to 80 psi. cold - dropping to hot 25 psi. at idle. Appears the problem is SOLVED, but NO, next day problem returns, 5 to 8 psi. cold until the RPM is raised to over 1500.
Is the oil pump pressure relief valve sticking part way open, on at startup? I also noted a small thread in "valve" inside the sump right above the oil filter, I assume this valve functions to bleed off any air before it goes into the filter, please don't tell me this could be the problem. I really don't want to take off the pan again. If it is the pump oil pressure relief valve would I be better off ordering a new pump and be done with it or try to replace the relief valve and spring?
Just call me nuts with it.