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Oil pressure question


NAXJA Forum User
Hi, everyone!

I have been reading along, found answers go most of my questions except this one...

First some info:

I have a 1999 2 door cherokee xj 4.0 auto.

I know very little about the history, other than gaskets were changed, Thermostat replaced after sticking closed. Heater core was blocked, I flushed the system and have been working on getting all the maintenance caught up. Odometer shows 144k, but was told carfax shows over 200k (haven't that verified myself). Most recently I found the cause of a mysterious coolant leak to be a cracked head, 0630 casting. Was losing maybe 1qt every 100-200 miles. Oil pressure was great (around 25-30psi at hot idle, 45-50psi driving) until about a month ago when I found the crack. Dropped to about 11psi hot idle and doesn't go above 30-35psi (hits that mark around 2500rpm and won't raise anymore with increased engine speed) verified via mechanical gauge... during the head replacement, I noticed a couple rocker bridges were scored a little and didn't use them. I replaced the head, been driving it for about 2 days as of now.

I know cam bearings are most likely the culprit. I am a mechanic of over 10 years, but currently dealing with a back injury and don't have a place to pull the motor... I also have little experience with jeep 4.0 issues. Everything is done in the driveway at an apartment with no garage. I also am not terribly worried about the oil pressure being an issue now, so much as I want this thing to last many years to come being a project me and my now 2 year old daughter have been working on together for the past year.

Now for the question:

There was tons of sludge, I couldn't get most of it out of the block but am running some flush through. Top end is getting oil. No knocks or rattles, runs great. With the oil pressure maxing out at roughly 30psi hot, is there any chance it's an oil pump issue? (Stuck relief?) I don't understand why I wouldn't get more pressure with higher rpm if it was strictly bearing clearances? Cold start I get 40psi idle, 50psi max raising rpm.

Also the rear main seal has been replaced before I got it, and i suspect it was done improperly like most of the other work I have been correcting.

Opinions? Advice? Anything is greatly appreciated!

If you think there's little chance of it being an oil pump issue, ill hold off going further until I can pull the motor and rebuild properly. If there's a decent chance it's the pump, I'll jump on it now for peace of mind.

I'm sorry for the long post, this is my very first time using a forum and wanted to make sure everything is there!

Thanks in advance!
All your info is a good evaluation, I would consider a new hv oil pump as a good investment in buying time. You might want to pull a couple of bearing caps while there. What weight oil are you using?
Thank you for the reply! I am running 10w30, currently cheap stp conventional high mileage for the flush but typically run full synthetic. I have read mixed things on the high volume pumps, although I have been leaning towards one anyways for a similar price to stock flow. I plan on pulling a few caps to check, if I end up pulling the pan... I'm mostly curious as to whether or not my symptoms could point to a worn pump or stuck valve? Being there was a bunch of heavy sludge and deposits, and the pressure only increases so much with engine rpm? I would think worn bearings would reduce pressure, but the higher volume with higher rpm would typically react similar to a high volume pump. What are your thoughts?
Not to likely on the stuck PRV, I would also step up to a heavier oil.
Thank you again! I was thinking about heavier oil, but was hoping something as simple as a pump would potentially be a proper fix...

I did notice oil at the bottom of the bellhousing before replacing the head, but the head gasket has been replaced before (had a victor reinz gasket, and the head/block look to have been resurfaced comparing to the replacement head with original surface). There was some oil seepage at the gasket, and I just got it all cleaned up yesterday to make sure it wasn't running down... could the rear main seal be a contributing factor, or is there a possibility any of the steps along the way of replacement could cause lowered pressure if not done properly? I have done one once, about 9 years ago and can't remember what was involved...
The rear main seal wouldn't effect pressure unless it was severely torn and you see that leak!
Unfortunately the oil pump feeds the cam first so you are correct about it being notorious for the problem.
I dont think that if it is still leaking it's that bad of a leak, so I'd say question is answered! It's just been wet underneath, blowing back along the tranny pan and mount. Also don't drive it far, but haven't noticed oil level dropping in the 2k miles we put on it since getting it (likely because it was filling with coolant!). Never saw anything on the ground after parking it, or the many times I've had it idling though. I guess I'll save my pennies up for the hv pump, one less thing to buy when rebuilding in the future.

I'd like to not modify/change the oil pan as im trying to keep this thing as close to stock as possible besides a few bolt ons (lights, winch and offroad bumper, etc. that came with it when i got it.) I've read mixed things on some people using the stock pickup screen, is there any downsides to that? I saw somewhere that there could be an issue with cavitation? I'll do what I absolutely need to do, but if the stock tube will work without any possibility of an issue, I'd much rather that route.
I've never heard of any pick-up or cavitation issues but I can see a possibility with the pick-up to pump entry. I clearance my tube entry. You will need to clearance the pan/pump for a hv pump.


I definitely understand and agree oiling properly is important, but beyond stock and getting into performance mods I am pretty lost ��. Luckily this isn't a daily driver, won't see much offroading unless you consider dirt roads and utility roads offroading. Mostly 55mph highways where pressure stays around 30psi. Working on making it car show material for the wife lol.
I'm not too concerned at the moment, and will switch to higher viscosity oil after the flush to get by another few thousand miles over the next couple years and focus on a rebuild then. Hopefully it won't chew the cam and crank up by then! (Good excuse to follow suite and stroke it though!) ��
I'm looking to pull the oil pan some time in the next week or 2 to check the bearings, decide what to do with the oil pump and have some more questions I hope you can answer or post a link to an already existing thread on it?

I've never gone into depth with engine rebuilds, it was always cheaper to replace them when I was working lol. I have done everything else imaginable on all sorts of vehicles.

I've been reading about checking connecting rods, but not so much how to check them when replacing the bearings for out of round? This is assuming I find obvious wear after plastigauging. And assuming I change the main/rod bearings while I'm in there, and the oil pressure doesn't improve, will the new bearings experience accelerated wear?

I've also been tossing around the idea of replacing cam bearings in the bay, possibly making a puller for the rear bearing or leaving it go if it doesnt look too bad. Have you heard of much success with that?

I do plan on doing a rebuild down the line, but if I can get a few more years and save the cam/crank while I'm at it, keeping everything all original as I've mentioned, I don't mind the few dollars spent and time it will take. It's easier this route for me, being I can usually only get to it at most an hour at a time a couple times a week and don't have a garage or accessible hoist at the moment
You asked a lot, but less than can be done with it in the jeep and assembled. I would just give it a visual to start and unless you see something scary leave it alone until you can do everything properly.
I understand and agree, but wanted to get a jump on info before digging in or the thread sitting too long. I'm half expecting to find worn bearings though and wanting to do everything I can to save what parts I can while I'm already in there. Plus, I'm really enjoying this learning opportunity! (So is my 2 year old who can already name and use most of my hand tools!)
These numbers are a little low...but honestly you're fine at 11 psi. Yea the bearings are probably a little worn but that's what you get with 200K miles. Its a 4.0 and it will keep running a long time with 11 psi at hot idle. If I was you I'd run it until it blows up and not worry about it until then.
Honestly, I'm thinking the engine was rebuilt or replaced at some point. The block looked like it was shaved, lowest compression reading I got was 135psi on cylinder 3. The rest were 145 and up. Didn't really have any ridge when the head was off.

My main concern with the low pressure is that it was maxing out at around 30-35psi around 2500-3k rpm hot, and wouldn't go any higher no matter how high I Rev it.

I just changed out the oil the other day to see what came out with the seafoam I put in, had 200 miles on it. Didn't look terrible, but I did have better pressure cold after the new oil was put in (gained about 5psi) and hot pressure was about the same as before. Threw about 2/3 bottle of Lucas oil stabilizer in and got about 2-3psi increase at idle, and I can get about 40-45psi at 3k rpm or so.

I dont want to run it until it blows, id like to be able to get away with just changing some bearings when I get a chance to pull the motor. Machine work to the cam/crank/block at that point won't cost me as much as a replacement, head was just gone over. I'd know what's in it at that point, and trust it more than the local used motors...

Anyways, I think the failure was all caused by lack of cooling system service. Thermostat stuck closed, cracked the head, caused sludge in the engine and wore something out. I was just hoping to get a proper fix for even less money now (without pulling the engine), than wait until it blows and figure it out then
I saw a significant bump in oil pressure at idle and off-idle when switching from 10w-30 to 10w-40. However 10w-40 is getting hard to find in a quality synthetic. Amsoil is one of the few that still provide it. Quaker State Full Synthetic Euro is available in 5w-40, and its supposed to be among the best of the full synthetics.