Questions and Answers

Jeep Low Oil Pressure Q. Vincent, I enjoy your site and have visited it on a several occasions. I have a bit of a unique situation and thought I'd try and get your opinion. My 1996 Jeep Grand Cherokee is indicating very low oil pressure at idle rpm. At anything above idle it "perks" up to 35+ and runs around 40 to 45 at highway speeds (depending on temperature).

When stopping at an intersection it slowly falls off to about 25 or 30 psi, then seems to hesitate there for a moment and then falls low enough to cause the "Check Gauges" warning indicator and beeper to activate. I presume this is probably a pressure below about 12 to 15 psi when this occurs. At this point just the slightest tap on the gas will bump up the rpm and it recovers to at least 30 psi.

Oil and filter are clean and fresh. The car has about 103,000 miles on it now but the engine doesn't burn any oil to speak of and it runs smooth and quiet. I've reviewed the article below by Carley but it seems to suggest that many oiling problems would show up at higher rpm, especially those on the "supply" side of the pump. Perhaps I have a pressure side problem with this engine. I'm suspecting a worn pump or possible a bit of trash holding the pressure relief valve open just a bit. I'd certainly appreciate your thoughts and opinions on this.

TROUBLESHOOTING LOW OIL PRESSURE
by Larry Carley, Copyright 2000 CarleySoftware.com

"The first indication of trouble may be a flickering oil pressure warning lamp..."

Specs:
1996 Jeep Grand Cherokee
4.0 litre, straight six,/ injected, non turbo
Automatic transmission
103,000 mileage

A. I chose to include Larrys article in the link library because he has got the causes of low oil pressure down pretty good. There are many causes for low oil pressure but the most common is engine wear. As Larry explains, as the engine wears and gets sloppy, the pressure will drop. But this is a gradual process, it doesn't happen all of a sudden. if the oil pressure loss is sudden one, then we have to look at the oil pump and pressure relief valve.

It is possible the pressure relief valve has some junk holding it partially open or the spring in it is weak or broken. Only taking it apart will reveal the answer.

But before I do any of that, I would use a mechanical pressure gauge and get an accurate oil pressure reading. It could be a bad oil pressure sending unit or gauge giving you a wrong reading. If there is valve or engine noise when the pressure drops, then chances are it is a pump problem.

One thing that will help is to use a heavier oil in the engine. This won't cure the problem, but it will help for a while until the root cause can be determined.

Additional Information provided courtesy of ALLDATA