There seem to be no shortage of XJ's whose oil pressure drops dramatically when fully warmed up, mine included. Most report that cold pressure is fine, north of 45 even, and then disappears to 12-18 when all the way warmed up.
Question for discussion is, is there an issue, something central to the XJ's only maybe, that causes this?

Does the AW4 have anything to do with the engine bearings wearing prematurely?
Does the temperature the XJ's cooling systems run at have anything to do with it?
Could the lifters or cam setup be causing the lifters to pump too much oil when warmed up?

..

 

Not really true, depends on maintenance and such and mostly overheating causes the pressure drop imo. but generally most of the jeeps with a 4.0 that come into the shop they also have lower oil pressure as well, but also a wrangler was one of them and the pressure on the gauge otherwise was too low the check gauges light was flickering on and off.

 

You have to keep in mind that the 4.0L oil pressure sending units are not very accurate. I learned that first hand recently

 

Minimum oil pressure should be 13psi@ 600 rpm.
Normal oil pressure should be 37-75psi above 1600 rpm. Oil pressure relief is 75psi.
You should be fine.

 

I had a few Ford 400M engines back in the 90's which when warmed up the oil pressure would drop to like 4psi at low idle (in gear with foot on the brake)! Obviously not ideal.

Another reason is the Jeep 6cyl typically idles a little lower than some other engines.

 

I guess I am one of the few who havent had oil psi problems. My last xj, 165k miles, idled warm @ 30 psi. The one before that, which I rebuilt the engine, idled at the exact same pressure. One was a aw4 one was a ax15. So I am pretty sure it has nothing to do with the trans. Oil temp is more important than coolant temp or trans temp when talking about bearings. Highly doubt it's the lifters either, unless they are bypassing where the plunger meets the body of the lifter. Cam bearings seem to wear out faster than anything.

 

Yeah, I have 2x Jeep I-6 motors one, a GC with almost 300k, both have good oil pressure at idle too.

I'd put a good gauge on it to double-check and keep yer eye on things. It could be, that the gauge had air in it that wasn't bled out. Really matters what the pressure is at the RPMs you use most.

 

In my '00 I am only getting 22 at a very hot idle. LOL.

 

It's the banjo bolt causing a restriction in the oil filter adapter. The oil flows thru the bolt and then exits thru 2 small side holes, around a gland, then into the filter. Try eliminating the adapter using a Wix 51348 or 51348XP screwed directly into the block. This is going to be my next step in solving this problem. Pressure is not a problem when engine rpm creates enough pump output to overcome the restriction. My WJ has the filter screwed directly into the block and has never showed me less than 40 psi ever!

My XJ holds 40 psi at idle hot at 1000 rpm, but only 25 psi at 600 rpm. I'm also using Rotella T6 5w-40 Synthetic. A much thinner oil, like a 5w-20 conventional oil may be better at holding pressure.

Anybody have a problem with the AW4 pump whining in drive when it's really hot? Just in drive, not at idle.

 

I get roughly 25 on hot idle myself in my 95 (with a 94 engine). My 00 TJ would hot idle at roughly 20 throughout the 300k miles I drove it, but I never checked the accuracy of the dash gauge. Both manual transmissions.

 

Hmm i keep forgetting about this, Gotta take a lookie at mine.

 

Thinner oil will lower your oil pressure, My oil pressure increased 10 psi when I switched from 10w30 to 15w40.

 

I noticed a long time ago that my oil pressure would run in the 60 psi range through whatever amount of load I had on the engine. Then after an oil change (always a Mobil 1 synthetic product) it would be responsive again for a little while.

With a new sender, it is properly responsive. Even after warmup it gets down to 20ish at idle, but back between 40/50 during driving. Guessing the above is a symptom that the sender is going bad. It is definitely gone when it is registering above 60 and erratic. Recently replaced a sender that failed and was blowing oil out the end of the plug, right down on starter and I guess aerosoling all over the engine.

 

It does seem heat related, which of course is when oil pressure drops anyhow right? Maybe the XJ runs hotter? Idk, my ZJ seems to hold around 30-35 or so at hot idle, but the XJ goes south of main street to like I said somewhere around 18, even tad lower if driving for a while. But always over 50 at startup.

 

Now this ..you may be on to something. However the banjo bolt is also on my ZJ, right? And that holds 30-35.
You're talking about the hideous T45 bolt, right?

Haven't noticed the pump whine on my XJ but mine was howling on the ZJ, then it just stopped..Only in 1st gear...Was like that for ages..