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4.0 oil pressure drops @ hot idle

Y

yossarian19

NAXJA Forum User
Location
Grass Valley, CA
Hey guys, I've been noticing the following on my 98 4.0
At startup it hits ~50lbs of oil pressure based on the dash gauge. While driving around town, 35-45 depending on RPM. When I get up to operating temp & keep it there a while, I'll notice a slow but predictable decline in oil pressure while idling, normally taking a minute or so to drop down to ~10 lbs where it will hold.
It has been doing this as long as I can remember, really.
The head tech around here thinks it is probably oil sump seals / oil pump pickup.
On the tech boards, guys are saying "cam bearings", though most of those cases are on 99+ engines with cracked heads (antifrieeze: 3. Cam bearings : 0)
For my part... dunno. Could pull the pan & do my rear main seal, check cam bearings while I'm at it, but...
I'm running Mopar filters & 10w30 with some Rislone mixed in.
When I ran 5w20, it did the same thing though with a slightly quicker decline to ~10 lbs.
Any other common failures?
 
14psi is the lowest nominal oil pressure at hot idle as per FSM, my 01 has been there for 2 years since my cracked head replacement. make sure you didn't blow a head gasket and have coolant in the oil.

i run 10w30 with 1/2 quart lucas stabilizer and mobil 1 m1-301 or royal purple equivilent filter
 
Monster Mopar said:
Have you checked the oil pressure with a mechanical gauge?? That's the first thing that should be done before tearing stuff apart and replacing parts.
Click to expand...

This.

Also, how many miles are on the motor? My '92 with 225k has similar numbers to yours except at hot idle it doesn't get lower than 20 psi. If you can verify that you only have 10 psi then that is a problem.

If it's borderline, I suggest running a thicker oil than 10w30, at least during the warm weather months. I use Rotella 15w-40, with one quart of lucas oil stabalizer (super thick) and one of Marvel Mystery Oil (super thin). This is good for a few extra psi compared to 10w30. It's not fixing the problem, obviously, but it will buy you time to prepare for a rebuild or whatever you need to do.
 
X3. Verify with a mechanical gauge. Can be rented from many big box parts stores.

Good data = good decisions.

I've seen quite a few 4.0 engines run at 10 psi for tens of thousands of miles. Don't over-react even if you are at 10.
 
Good point, all - the funny behavior of the needle (back & forth on its way down to 10) and the oil sender being a cheapo unit doesn't speak well for the data. I'll verify measurements before tearing down the pan & checking bearings.
Hell, I only drive it ~6K a year anyway.
 
Probably time to replace the oil pump. They go bad in the 4.0's. Not to hard to change either, good time to do the timing chain and RMS as well. Get the Melling HV pump. You will be happy, just be sure to chunk the pick-up that comes with it unless you want to bend it to fit your pan.
 
nblanton said:
Probably time to replace the oil pump. They go bad in the 4.0's. Not to hard to change either, good time to do the timing chain and RMS as well. Get the Melling HV pump. You will be happy, just be sure to chunk the pick-up that comes with it unless you want to bend it to fit your pan.
Click to expand...

Great advice but best to VERIFY that ther truly is an issue with the oil pressure before jumping on that bandwagon..
 
The high pressure with cool, viscous oil and subsequent low pressure with hot, less viscous oil pretty much verifies the issue. I've seen the exact same symptoms in two 4.0L in the last year alone; both were fixed with a new oil pump.

I suppose it could be a sloppy bearing, but that would seem rather unlikely. Honestly, there isn't any way that I can think of that will verify that the pump is not supplying sufficient pressure due to internal wear or a bearing that has gotten out of tolerance. You would need to have some other symptoms to imply that the bearing has gone, like noise or non-magnetic metal in the filter media. Or a piece of plastigage, which would only work with the crank and rod bearings, not the cam bearing that the OP was informed it may be.
 
Perhaps I'm missing the point here, but I've never seen an xj with oil pressure as measured with the gauge that didn't have high pressure with cool oil and lower pressure with hot oil. How does this verify that it is the pump?

The OP has confirmed that he will check with mechanical gauge -- pure speculation until that occurs.
 
Those symptoms sound like what I had. It ended up being cam bearings, but was a recent rebuild so it wasn't stock. Not saying that's what you have, but if it turns out you are losing oil pressure at hot, it's definitely something to check.
 
Monster Mopar said:
Using thick ass oil doesn't "solve" any problem when it's not originally called for in that particular application... It only masks the problem and prolongs the inevitable
Click to expand...


That is exactly what most people want in an old ass engine, to prolong the inevitable for as long as they can.

My XJ have 235+ miles on it. I ru 20-50 Amsoil and agood quality oil filter. The idle pressure is 30 psi when cold and 60 psi when up to speed. It look like an oil with a higher viscosity that 10 could well buy the OP some time/help him prolong the inevitable.
 
Monster Mopar said:
Using thick ass oil doesn't "solve" any problem when it's not originally called for in that particular application... It only masks the problem and prolongs the inevitable
Click to expand...

I always ran 20/50.... I used to run Fram filters and I had the low pressure at hot idle.

Once I switched to the Wix filter it idles at 20 all day long.
 
Lol My 215k 87 with who knows how many miles on the "rebuilt" 88 engine with a screwed bottom end runs ~40psi all day long even at 300rpm idle. Don't ask me why it idles so low it's been creepin down since I got it plus when I bored the TB for some reason it dropped some. Runs 400rpm normally.
 
Just a few questions for some of the Jeep 4.0 engine specialist out there,

Is 10Psi hot idle with a quality 10W 30 oil such a bad thing?
Is it that much worse than the 13Psi hot idle spec that it needs to be repaired?
I have had 10-11 psi hot idle oil pressure using 10W30 M1 or Pennzoil Platinum for years now on my 140K 2001, It increases to about 15psi if I use 10W40 M1 or 5W40 M1 turbo diesel oil, higher if I go to 15W50.

It does seem to run smoother/quieter on the 10W30 & it does not burn a drop between 5K oil changes, Oil also stays nice & clean (I only run the larger M1 filters).
 
Last edited:
After trying Castrol Syntec, Edge, Mobil 1 (JUNK!!!), and Valvoline, 5w-40 Rotella T6 fixed my lifter tick issues and brought the oil pressure up a bit. As long as the lifters aren't ticking, I don't much care how low the oil pressure goes. When working at the barn pulling trailers around the field (aka- hot oil temps, hot coolant temps, and cooking trans temps), it'll drop down to the 10 psi mark, but it never makes a peep. I've actually switched to T6 in both the toys (the LS1 Miata being the other) and its silenced the sewing machine in it also.
 
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