texmedic49

I noticed about 2 months ago after doing a routine oil change that my pressure had suddenly jumped to 60 or so on start up. I didn't think much of it, but it continued to happen. Before that oil change, my pressure was a healthy 20-40, and never really budged. What concerns me is the fact that it changed drastically after one intervention, and not gradually. Now before everyone jumps on the "that's a great pressure" bandwagon, I am focusing on the change (and possible causes) not necessarily the pressure. I HAVE verified the start pressure with a manual pressure gauge, which showed 62 psi on start. I am also aware if the FSM pressure ranges.

I am concerned that the sudden change could indicate blockage of some variety, which I don't think is an unrealistic concern. The finding that I feel supports this concern is the fact that the pressure will slowly decrease as the engine heats up, and is the "high" pressure is not constant. I know about heating expansion and all that.

Another semi-concerning finding is that the pressure stays around 60, or sometimes greater while driving at any speed or tach reading, and will drop to 20 or so when I let my foot off the pedal to turn, slow down, etc.

My plan is as follows:

1. Change the filter to another brand, on the off chance that I got 2 bad Wix filters in a row, or they have been sitting up too long in my garage. I usually buy 2 or 3 at a time.

2. Recheck pressure after the change.

3. If still consistent with the initial findings, run MMO through it for a few trips to work and back (20 miles or so) and see what happens.

4. Another complete oil and filter change after the MMO.

5. Recheck pressure after the change.

6. Possibly remove valve cover and poke around through the galleys, but need to do some more research on risk vs. benefit of this.

This 1996 engine doesn't have that many miles on it (~170K), and is a daily driver with the semi-frequent trips in the sand and mud. Nothing extreme.

I've read a lot about spun bearings, and I'm trying to narrow down any other possible causes for the sudden jump in pressure. I am also trying to game out the length of time that I might have to pull another long block out of the junkyard and work on rebuilding it, if there is indeed an issue.

In my profession (medicine) you tend to focus on your last intervention as the cause of changes that come up quickly, and that's the way I am trying to solve this.

Does anyone have any other ideas, or something that I have left out or not thought of?

EZEARL

62psi verified with a mechanical gauge is the only value I'd go by. The sending units(even oem ones)just aren't accurate. I'd hook up a mechanical gauge inside to see what pressures your getting during all vehicle operations.

texmedic49

Thanks for the reply, I'll have to research that. I guess another avenue to assess is the possibility of leaving the manual gauge hooked up, letting the engine come up to operating temp, and then test the theory that the pressure is indeed spiking by monkeying with the throttle. I would assume that this would simulate driving adequately, but maybe I'm wrong.

I also didn't state in my original post that I was also trying to verify the (unlikely) accuracy of the sending unit with the manual gauge. Some data is better than none at all.

EZEARL

I should have mentioned that even with my MOPAR unit I sometimes get a variation in the readings. Not much but there. My start up reading is never less than what your mechanical gauge shows.

I'd hook up the gauge and test drive it for a bit.

texmedic49

That does make me feel a tad better. What is confusing is the fact that this SUDDENLY happened. I'm focusing on the possible causes, assuming the data are correct. I will continue to confirm the pressure readings, but have no reason (right now) to doubt what I'm seeing. The electrical gauge reading was confirmed by the mechanical, give or take a few psi. Granted, this was on start up, only for a short time, and not during operation.

EZEARL

Just to eliminate a possible problem check the connector on the sending unit. Clean it with spray contact cleaner,apply some dielectric grease,and check for a good fit. After all you were around it when you replaced the filter.

texmedic49

Already done. I replaced the sending unit as well when it happened, as that was the most obvious explanation.

Fsher21

Here is another possibility to consider. Those sending units are quite sensitive, especially if they get tapped while the oil filter is being removed. There is a chance your sending unit was off when it was reading at the 20-40, and it got bumped during the last oil change so that it read more accurately, which was then confirmed by your mechanical gauge. A pressure of 20-40 is kind of low for a cold start, I don't expect to see it that low until it is up to temp.

tl;dr-maybe it was wrong from the start, and is now correct

EZEARL

I know each motor can be different but my '96 4.0 w/142k has oil pressure readings of just below 40 for a low and 60+ for a high. He's in the ballpark reading 60+ w/a mechanical gauge. I'd like to know what his idle rpms are with one.

texmedic49,did you replace your sending unit with a MOPAR product?

texmedic49

I had considered that possibility as well. But after several days of mental gymnastics, I figured out that I had replaced the sending unit probably 3 times due to common failures over a 5-7 year period. Kinda hard to remember exactly, and I hadn't started keeping a diary of vehicle repairs yet. I do now.

The sender is not Mopar. I am embarrassed to admit that I replaced it with Napa. I've had good luck with their house branded stuff, as most of it comes from the major manufacturers anyway. I recently found out that Monroe makes their shocks and Wix their filters. Who knew?

As for idle RPM...800ish comes to mind, probably wrong though. Don't take that as a solid answer. Its raining like a bastard right now, so I'll have to get back with you on that once it clears up.

I sat here and thought about it some more, coming to the conclusion that it very well could be the filters. I bought the last 3 that I used in one go, and they sat in my garage for a little while until I figured out that I had them. I used new off the shelf ones in the interim. Does the possibility exist of hardening of a drain-back valve in the filter itself? I admit that I haven't paid too much attention to the way the guts of an oil filter work, figured there wasn't really that much that could go wrong. Just throwing that out as another possible source of restriction that might increase pressure.

Thanks again for the replies.

EZEARL

Sorry. I meant I'd like to know what your oil pressure at hot idle (not rpms) is with a mechanical gauge.

I really doubt your filters are the problem. Maybe if they were contaminated during storage or such.

One thing that is mentioned quite often on this site when the subject of replacing sensors,sending units,relays,etc for the Cherokee electrical/computer system is that for the best performance, reliability, and accuracy it's best to replace them with MOPAR. Seems that those in the know here say these systems don't work well with aftermarket replacements.

texmedic49

So I wasn't far off...a little over 600 after it settles down, assuming each mark under 1000 is 333. I never thought about it, but that is very inconvenient. All the rest are 250 per mark. I wonder who came up with that?

I'm going to hook my manual gauge back up this afternoon and let the engine come up to temp, and take readings about every 10 minutes and see if there is a curve that develops. I'm pretty sure there will be. I'll also press my loving wife into service to step on the gas so that I can get values at different RPMs. I just hope my Harbor Freight gauge is up to the task and doesn't blow out on me.

added: You and I must have been typing at the same time. I'll post the curve later tonight. The cold idle value still helps though. Thanks again!

colt44

I've always changed my own oil and I've had my XJ for 19+ years. These things have what is called a "active oil pressure gauge" which means it's a lot more indicative of what's really going on in motor, as apposed to my Ford Mustang that goes half way up and stays there all day. A couple days ago, I drained my 5w-30 and put in some 15w-40 Royal Purple for the summer. The oil pressure hanged at between 40 and 60 {according to factory gauge] for a couple days and then stabilized at about 35-40 at idle and went up from there to about 60 at higher RPM's. That's good PSI. When my oil pressure sending unit went out a couple years ago, It would go to 90 psi ! But I replaced it with a NAPA part and it has worked good. I remember reading something years ago that said your oil pressure should read 10 psi for every 1,000 rpm, but never drop below 10 psi. I believe the addition of fresh oil, with higher viscosity may be the cause of your concerns. Wait until a 90 degree day and see if your pressure drops to 35 or so. [even 10 psi is acceptable on a hot motor at idle.]

texmedic49

I failed you Earl...

Things went sideways last night and I didn't get that info. We all got a little tight around here, and I thought the better of trying to work on the Heep while slightly altered. I like my fingers. I'll get that done today.

EZEARL

Better safe than sorry!