Oil Pressure Problem...

I have a 1991 4.0 - 232,000 miles on the vehicle, around 100,000 miles on the engine (bought used with around 87,000 on it).

When I start it up the oil pressure is around 50lbs. As I drive, it drops, to the point where I'm only at around 15 - 25lbs at 65 MPH on the highway.

Coolant temp is a hair over 210.

I've replaced the oil pressure sending unit (no change).
I'm running Mobil1 5/30, using a Wix filter.

The engine sounds good with no knocking, rapping or noticable loss of power. I'm not losing any coolant.

Any ideas?

Just an oppinion and a few observations. Some manufacturers used to recommend going one oil viscosity (wieght) value higher in an older high mileage motor.
Synthetic oils are labeled with a relative viscosity rating. They are usually thinner from the get go, but don't get as thin as fossil oils at operating temperture. It used to be a 5 wieght synthetic was roughly equal to a 30 wieght fossil oil. They started lableing the synthetics with a relative viscosity rating to avoid confussing the customer.
As the cam, crank and other parts wear, the oil pressure often goes down some. I usually go to a higher viscosity oil. 10-40 Mobile 1 has always seemed to work well in my XJ's.
5-90 mentioned awhile back that the adhession enhancers that used to be found in many oils have been reduced (this also affects oil pressure) to save fuel. Much of the same reason (to raise the fleet average fuel consumption statistics), modern motor manufacturers recommend using a lower viscosity oil, than was recommended twenty years ago.
I've never noticed much difference using a summer rated oil in the winter (especially a synthetic) until the temperature gets well below 0 F.

I've been running 10W40 in my 4.0 all winter, seems just fine.

Rootehound said:

I have a 1991 4.0 - 232,000 miles on the vehicle, around 100,000 miles on the engine (bought used with around 87,000 on it).

When I start it up the oil pressure is around 50lbs. As I drive, it drops, to the point where I'm only at around 15 - 25lbs at 65 MPH on the highway.

Coolant temp is a hair over 210.

I've replaced the oil pressure sending unit (no change).
I'm running Mobil1 5/30, using a Wix filter.

The engine sounds good with no knocking, rapping or noticable loss of power. I'm not losing any coolant.

Any ideas?

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Since you've already changed the gauge sending unit with the same result, it's highly likely that the oil pressure really IS low. You could verify it with a mechanical gauge but I doubt it'll show higher readings.
I suggest you remove the oil pan and change the oil pump. With the high mileage that it's done, it's probably worn. The pick up screen could also be clogged up. Clean the oil pan thoroughly before you bolt it back on again so that the new oil pump doesn't suck up more dirt into the pick up screen.
While you're there, check the rod and main bearing clearances with Plastigauge. If they're more than 0.0025", the bearings will need replacing.
Hopefully you'll have better oil pressures when all's said and done.

I have the same problem. I put a "T" in so I could have my normal oil gauge hooked up and a mechanical one hook up at the same time and the mechanical one always read higher. I hooked up a scan tool up to the computer and it was reading the same low pressure that my gauge was reading. The only thing I can figure it that the computer is misreading the signal from the sending unit. But I don't know how to fix it.

91 recommended viscosity is at least 10W30, it wasn't till 00 or 01 that they recommended 5w in the jeeps and that was a CAFE thing...wifes oldsmobile runs 5W30 and that stuff just makes me nervous, 0W30 even more so as it's 'recommended' for winter weather.. to me 0W = water...

Newer engines are being built to much tighter tolerances (bearing clearances are becoming as tight as 0.001") so they can run thinner grades of oil and smaller oil pumps while still maintaining 40+psi oil pressure. The combination of thinner oil and a smaller oil pump results in less parasitic HP loss and better gas mileage, so that's the reason for the tighter tolerances.
The Jeep 4.0 engine should have bearing clearances in the 0.0015-0.0025" range with 0.0020" preferred.

Thinner oil and smaller oil pumps may give you less parasitic horsepower loss, but building an engine with tighter tolerances only negates those very small advantages. When we build our Mopar engines for drag racing, we actually use loose clearances to free up horsepower. Its my understanding from some others here, that the 4 liters were put together relatively loose also. Maybe not as much as a race engine, but enough to get by on the street day to day.

Thanks to all!
I'll change the oil this weekend and use 10/40, see what happens.
Hopefully I'm not looking at bearings.

If you do decide to do the bearings, you can do the main bearings and rod bearings from underneath, with the oil pan off. You should also replace the oil pump at that time.

Use plastigauge on the bearing journals to see how much clearance you have, and that will tell you what size bearings to use.

And ... if you're feeling *really* ambitious, you can also remove the head for a valve job and ring replacement (with the oil pan off, you can crawl under the vehicle and push the pistons up out of the top). Just make sure you hone the cylinders. Kind of a poor-man's overhaul.

Good luck.

BCKNBLK said:

Thinner oil and smaller oil pumps may give you less parasitic horsepower loss, but building an engine with tighter tolerances only negates those very small advantages. When we build our Mopar engines for drag racing, we actually use loose clearances to free up horsepower.

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That makes sense in a drag racing engine but such an engine only needs to last just over a 1/4 of a mile. How much oil pressure do your drag engines generate?
We want our Jeep engines to last well over 100k miles so they have to generate good oil pressure (at least 40psi from 3000 rpm upwards). There are two ways to get there. Either looser bearing clearances and a high volume oil pump, or tighter bearing clearances and a stock oil pump. The oil should be 10W-30 unless you're running in very hot weather (15W-50) or very cold weather (0W-30).

Dr. Dyno said:

That makes sense in a drag racing engine but such an engine only needs to last just over a 1/4 of a mile. How much oil pressure do your drag engines generate?
We want our Jeep engines to last well over 100k miles so they have to generate good oil pressure (at least 40psi from 3000 rpm upwards). There are two ways to get there. Either looser bearing clearances and a high volume oil pump, or tighter bearing clearances and a stock oil pump. The oil should be 10W-30 unless you're running in very hot weather (15W-50) or very cold weather (0W-30).

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Sorry, but have you ever poured out a 0w30 oil, stuff is like water to me...5W is almost as bad and I use in the wifes olds with it's Jasper reman engine. I've been real happy with 10w30....

RichP said:

Sorry, but have you ever poured out a 0w30 oil, stuff is like water to me...5W is almost as bad and I use in the wifes olds with it's Jasper reman engine. I've been real happy with 10w30....

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On a 20*F day, you'd be glad to have thinner oil so that it reaches the top of the engine faster at cold start-up. That said, 10W-30 is OK from 30*F to 100*F so it'll do the job virtually year round.

This is all helpful stuff, but it leads me to another question...

The engine in an XJ with the AW4 is only doing around 2000 RPMs at 65 MPH.

If Chrysler states that the vehicle should have 40lbs of oil pressure at 3000 RPM, is it acceptable for it to be 5 - 10 lbs lower at 2000 RPMs?

Is there any "table" of acceptable oil pressure at RPMs?

Never seen one, because oil pressure varies so much with conditions, especially temperature.

Take the motor up to 3000rpm, if you get 40PSI or better your good.

As far as the discussion about tighter tolerances and thiner oils. One thing to note about using higher viscousity oil in new tight clearance motors. I put an Oil Pressure Gauge in my '99 Neon R/T when it was new. Also being of the impression that higher viscousity will always protect better, I also was using 5W-50 Syntetic Oil, as soon as put the gauge in, I noticed that starting the motor on cold mornings, like 50°F, my oil pressure gauge was way over max spec oil pressure like 75PSI. Driving at anything over 2500rpm, while the motor was cold, the oil pressure gauge was pegged (it maxes at 100PSI). When the motor was warm, I got good oil pressure, 35PSI idle and 60PSI off idle and there in all conditions. I was worried I'd blow seals with the 100+PSI, so switched to the recommended 10W30 oil and that was much better. Stayed under 75PSI while cold, warm it was the normal 27.5PSI at idle and steadily increased to 55PSI to 2500RPM and even in the 60's at high rpm.

So be cautious with using higher than recommended oil viscousity, especially in a new motor. Sure, more oil pressure is always better for protecting the motor, but if your making almost twice as much oil pressure as the pressure relief spring in the oil pump, I'd think you now have to worry about blowing seals. As well, the oil system is a balance of pressure and volume moving thru it. If pressure is too high, you have to worry if enough volume is moving thru the motor, that is cooling many of the internal parts.

There was a Harley and Indian engine builder in NJ where I lived. When he would assemble the engine he'd drop it in a large tub of machine oil and hook up an electric motor to the drive pulley with a chain or belt, then run it submerged in the oil for 12-24 hours, very slow speed. I was not into HD's back then but I understood that he was backed up several months building engines and they were in demand....

Rootehound said:

This is all helpful stuff, but it leads me to another question...

The engine in an XJ with the AW4 is only doing around 2000 RPMs at 65 MPH.

If Chrysler states that the vehicle should have 40lbs of oil pressure at 3000 RPM, is it acceptable for it to be 5 - 10 lbs lower at 2000 RPMs?

Is there any "table" of acceptable oil pressure at RPMs?

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No, no table, just the PSI @ PRM figure.
There is a "rule of thumb" that states 10lb per 1K untill factory spec. Most idiot lite switches go off around 3-6lb.

Let's break this down to simplicity. You have a high milage engine 187K (87K start=100K you put on it).
It ain't new.
It ain't close to new.
It isn't even close to close to new.
It is, in a word, tired, worn out, kaput (well actually thats 3 words).

My advice?
Be happy with it as long as it doesn't puke it's parts all over the freeway
Be thrilled with it every day it starts and drags you and it's tired arse to work so you buy a new one
Change the oil a bit more frequently, use 10-30W or 10-40W, whatever. Just don't use 20-50W in an attempt to get back lost pressure

I'm beginning to believe a great number of people have gotten their knowledge of oil and engines from "Paw" or books and not enough from practical experience.

What I meant was the engine had around 87,000 miles on it when I put it in (last summer), I've put another 13,000 or so on it, so it has around 100,000 miles total on it.

It's been my experience that 100,000 miles on a 4.0 is still "low milage". (A 1990 I once owned had 210,000 on the original engine and was still running strong until some drunk in a T-Bird t-boned me one night).

Before I get into doing bearings I was curious as to anything else that could cause the oil pressure problem.

Rootehound said:

What I meant was the engine had around 87,000 miles on it when I put it in (last summer), I've put another 13,000 or so on it, so it has around 100,000 miles total on it.

It's been my experience that 100,000 miles on a 4.0 is still "low milage". (A 1990 I once owned had 210,000 on the original engine and was still running strong until some drunk in a T-Bird t-boned me one night).

Before I get into doing bearings I was curious as to anything else that could cause the oil pressure problem.

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LOL! I was never good at story problems

I have a 1991 4.0 - 232,000 miles on the vehicle, around 100,000 miles on the engine (bought used with around 87,000 on it).

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I take back the "It's not even close to close to new" remark, it is still simply "not even close to new"
100K miles, and you have no idea what kind those other 87K miles were? Do you know if they were postal deliver miles, or highway miles, or both?
Regardless, 100K is still up there. True many XJ owners have many more miles on theirs than that (mine has 177K, mostly highway miles), but some don't.

I wouldn't even buy the bearings untill I did an inspection. If the journals are gouged, you will be wasting your time.

Zuki-Ron said:

...Just don't use 20-50W in an attempt to get back lost pressure

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? Just out of curousity, why don't you want to use 20W-50 to get back lost oil pressure?

Oil pressure is oil pressure, if you had bad oil pressure with 10W-30 and 10W-40, but 20W-50 gets you good oil pressure, why would NOT want to use it?