91 Cummins With Low Oil Pressure
 

A coworker of mine has a 91 Cummins that loses oil pressure mostly at idle after it warms up typically on long trips. He has an auto tranny and the low oil pressure does something to the shifting as well. The local mechanic is stumped so if anyone has any ideas I would appreciate it.

How many miles on the truck? Have you checked the sensor/ checked pressure with a mechanical gauge? Fresh oil or old oil? Correct weight of oil?

~270k on the truck. Mechanic checked the sensor and pressure. He just changed the oil, it is the correct weight. He uses lucas additvive, but has for a long time now. The mechanic replaced some spring near the oil filter, and it is better for now. I'll post on here again if it goes back down.

Low idle speed can cause low oil pressure at idle. The oil pump is designed to pump a certain pressure at a given RPM. Drop below that RPM, and you have less oil pressure pumped. Make sense?

Why is he putting Lucas additive in the oil? I've had 2 of them that still had cross hatching in the cylinders at 270K or more. 5K oil changes on dino oil and decent (Wix) filters. Nothing special at all.

DP

I have no idea why he puts Lucas in. He just does what he wants. I posted the question because that is all I heard about at work for a week. He claimed that he monitored the oil pressure very closely and it was very low with some trouble shifting. The truck is gone and now he has an FJ Cruiser so I am not worried about it. Thanks for the response though!

i bought a 91 with a 5.9 the owner thought was going bad because after it warmed up good the oil psi would drop to nothing at idle and the low oil psi light would come on the dash .. install a mechanical gauge if it does not have one and see what the psi really is "" and the low psi does not have anything to do with it shifting..

Low oil pressure 98 12 valve cummins 5.9l
 

I have new oil and
New oil pressure sending unit.
Any ideas why pressure acts up after truck gets good and warm?