4.0 has good oil pressure at start up. (40psi) when it warms up its fine while driving and then at idle it drops to 10 or a bit less. No lights ever come on. I replaced the pressure sensor and it did nothing. Please help.

 

Job #1 is to VERIFY.

Get a hold of a "mechanical" oil pressure gauge. You can rent them from many big box parts store.

Good data = good decisions

The oil pressure spec for the 4.0 is as follows:

*Not less than 13 psi at HOT idle

*Between 37-75 psi over 1600 rpms

Let us know what the mechanical gauge says. That will tell the story.

 

jeeps have always had a wide range in oil pressure. i had a cj 4.2 that showed less than 10 lb at idle and over 60 while driving. never had a problem. the chevy's i have owned usually had a 20 lb swing at the most.

that being said, there is a pressure relief valve in you oil pump. it is spring loaded and releases excess pressure to keep from blowing seals when the engine is at high rpm. it is possible that this valve is stuck open (maybe partially) and the oil circuit doesn't develop the pressure it should at idle.

 

Unless you are having an additional problem other than the psi showing up low on the gauge I wouldnt worry to much about it. The readings and timing you state is pretty much what mine has always done.

 

My '98 does displays identical gauge pressure as the book describes, only occasionally dropping to 10 psi but coming back up to 13-15. We've put 4K on since purchase with two oil changes and have not had any problems.

 

Get yourself a chiltons or something similar and read it please

 

The 4.0 Jeep engine of the '90's was notorious for the oil pump failing at 145 to 155 thousand miles. That said, you might check a few things first in order of frequency:

1) Check the most obvious - be sure there is plenty of oil with no leaks

2) Check the oil sending unit - replacement is quick and easy

3) Check the oil pickup unit - if you run Pennzoil it may be clogged

4) Check and replace the oil pump - they go bad around 145K miles


I'm not a mechanic, but I do my own work and hope this helps....

- Doc

 

The general rule of thumb is 10 psi per every 1,000 rpm. So your good. 40 psi above 1600 rpm is pretty normal for the 4.0L. 13ish+ psi hot idle is acceptable according the factory specs for the 4.0L , so your just below factory spec at 10psi but i honestly wouldn't worry about it. Just be faithful with your oil changes, always get a good quality filter, and refill with 10w30. If your oil pressure gets below 10psi hot idle and below 30psi hot above 1600 rpm, then yeah i would worry about it. Just as a side note, Jeep gauges even with a good sender are famous for being inaccurate. The only way to know for sure is to rent a mechanical oil pressure gauge, and test it that way.

 

I developed a low oil pressure situation myself today. Had been perfectly normal, than all of the sudden at idle, got the check gauges indicator. 0 psi oil pressure at idle after coming off the highway. Gauge normally rock solid at the half way point. No accompanying noises, and have driven additional 50 miles today. I assumed a sending unit failed. Decided to do a little research, and have found numerous people have found the brass adaptor that the oil pressure sender screws into can become obstructed. Some remove to clean, others have run pipe cleaner, or a piece of wire in to resolve.

 

Beware of the things that can happen when using Fram oil filters and other junk filters out there.

 

Mine has been 4psi @ idle and 20psi @ 1600 for years and over 50k miles