I recently got a 97 wrangler 2.5L. When its cold my oil pressure at idle is around 40 psi on the guage. It runs a little higher as I am driving. After it runs at operating temp for a half our the idle oil pressure grdually drops to next to nothing. It doesn't knock so I am thinking a bad sending unit. If i tap the gas it comes right back to 40, and then gradually goes back down.
Any thoughts?  

No solution, just a similar problem.  Have a 2000 XJ with 4.0L.  Oil pressure cold, and at highway speeds is 50 PSI.  However, once warmed up, when I stop for a light, pressure drops to approx 25-30 psi.  The motor has 60K miles, doesn't us oil, no other problems.  Any ideas on why the pressure (as measured by the dash gauge) drops at idle?  

First the 2.5, I think your on the right track...sending unit or a loose connection to it. Second,oil viscosity (thickness) is always higher when cold. As far as a lower reading at idle, the oil pump is mechanically driven so at idle it is spinning (pumping) at a slower rate, so you get a lower reading.  

I have a 2000 Wj and it did the same thing when i put 10w-30 in it. the oil psi was (0) when hot.I put 20w-50 in it and the oil psi is great now. about 30 at idle when hot.  

Be sure you are running the recommended oil viscosity for your engine. It is more important than most think.

On the 2.5, you are all on the right track with checking the sender.  

I also have '97 Wrangler with 2.5L. This engine does basically the same thing, when running high RPM's the oil pressure is good and stays up good most of the time at idle, but every once in awhile, say when I stop at an intersection and engine drops to idle around 900 rpm,it will drop another 100 rpm, and the oil pressure will drop to 0, I have to idle it up immediately. I replaced the oil sending unit and still the same. It is losing oil pressure, I've also changed oil viscositys and no help and I can't find anyone to give me an answer to this problem. So if you find out about your problem let me know....  

Older engines are bound to have some wear and looseness between parts. That's typically what causes low pressure. Just make sure you have about 10 psi for every 1000 rpm and your motor should run for a long time. Of course check the sending unit and wiring too.

John Cappa
Editor, Jp Magazine  

My '97 Wrangler 2.5 is another victim of low oil pressure.I've never seen it over 55 psi. Most of the time 20 to 40psi but at idle 8psi is the norm. It scares me. I'm thinking about installing a new pump just for my mental health. Could it be bypassing for some reason? It has 97K on it and I use oil for older engines. It seemed to improve the last time I changed the oil filter.  

My 95 YJ has about 170,000 on its 4.0, haven't had any problems with loss of oil pressure. I change the oil every 3 months religiously, always 10W30. This must be a trait in the newer engines, seems it's happening from 97 on up...Hope you guys can sort this out.  

I have the same problem with my 96 XJ (4.0)as Fiftiesboy up above.  My oil pressure at start up is usually around 60 but like you said after it is warmed up and has been driven for a while when i come to a stop it drops down to about 30 psi ....i have no idea why this is...anyone know or have similar issues with the XJ?  

I have the same issue with my 79 cj 4.2L. At start up 60 psi, once warmed up at idle it drops to a cozy 4 or 5 psi.as soon as you come off idle the pressure comes back. I double checked it with an mechanical pressure gauge. I was told it was probably the main bearings wearing, and to bump up to a hd 10/40W or 15/40W.  

A good thing to do to make sure you do infact have oil presure is hook up a mechanical gauge. you can find one at any parts house for $15.00 or under. This way you can verify when the gauge goes down does the presure also. This will tell you right away if its the sending unit.    

New bearings, good pressure
The engine had to come out in order to do the job right. We recovered the refrigerant, removed the condenser, radiator and grille. We took the powerplant out of the Cherokee and mounted it on an engine stand. The cam bearings hadn’t spun, but they were badly worn. The rod and main bearings looked just fine. Gaskets and bearings were all that would be required: The cam bearing journals measured within specs, and the timing chain had only a small amount of slack. Sliding the lifters back in their bores, we applied air and felt a good volume available at all four cam bearing ports.

The cam bearings on the 4.0L look almost exactly alike, but are selective in size and design. Thus, they must be installed in the proper journals. To their credit, my students discovered this peculiar design feature while checking clearances. The new OE cam bearings are actually color-coded and come with an instruction sheet, but the aftermarket bearings we ordered weren’t color-coded and came with no instruction sheet. My parts supplier called the company and was told that the part numbers stamped on the bearings are sequential, with the highest number riding in the rear of the engine and the subsequently descending numbers fitted for the other three journals in countdown order. With the new bearings in place and air pressure applied, we found a much tighter gallery than before. The leakage was gone.

With the engine reassembled and filled with new oil, a new filter and new coolant, the gauge indicated a cold oil pressure reading of 60 psi. After burping the air and recharging the A/C system, the students drove the vehicle for almost an hour. When they returned, the oil pressure was hovering at a hot idle figure of 40 psi.

Backtracking
We hadn’t found any concrete evidence of what had wiped out the bearings, so I called the tire store and spoke to Jeff, the tech who had replaced the oil pump. Jeff’s report was interesting.

When he had first checked the oil pressure with his master gauge, he had seen seven pounds at cold start and zero pressure with a warm engine. Removing the oil pump and spinning the gears, they seemed to be loose, and when he shook the pump, it rattled. He didn’t disassemble the pump or apply air pressure to the gallery. What was even worse was that the scrap metal man happened to be present and the old oil pump was sent to the scrap truck.

He replaced the rattling pump, filled the crankcase with oil and restarted the engine to find 50 psi at cold start. Letting the engine run until it was at operating temperature, he still saw a comfortable reading on the gauge (he didn’t tell me the numbers) and thought he had cured the problem. However, when the owner picked the vehicle up and drove it around town for awhile, the oil pressure continued to drop until it was near zero again.

Back to the rear view mirror
I had backtracked as far as I could and it appeared from the report I got from Jeff that the oil pump had been the causal component. The reduced pressure would have allowed the cam bearings to wallow out, and while the new pump helped somewhat, the bearings were already past the point of no return.

The lesson my students learned from this experience is invaluable: While replacing the entire engine is sometimes the best way to go, this 39,000 mile Cherokee just didn’t need a $5,000 replacement powerplant. Replacing the cam bearings would have paid a flat-rate man about 10 hours labor, and even at a shop labor rate of $100 an hour, the Cherokee could have been properly repaired for less than $1,500.  

found this on this site
http://www.search-autoparts.com/searchautoparts/article/articleDetail.jsp?id=66740  

Pump is mechanical. when the engine slows the pumps slows down. Have a 2000, when the water pump went i started having sever oil pressure problem. when running, good pressure, at stop it would red line.
If you water pump start to go and your gauge starts to rise, shut her down and have it towed. I ran mine watching to make sure it did not peak, water temp,
but i think the cam bearings were damage due to the extra heat. good luck.