Oz E

Hi Dave!

I should've mentioned... it is a 2001 XJ.
I have checked the trouble code already by doing what you suggested. It does say "Check Gauges".
The oil pressure is usually around 20 and when I start driving it it goes up to 40. I purchased the truck in May 2019 and have not changed the oil yet. I got the alignment done and started driving it only couple of weeks ago as I have been working on it all summer.
Maybe I should take it to a shop to see whats going on but you know they charge outrageously

EZEARL

Accessing the OBD using the key method will only tell you so much. Take your vehicle to one of the auto parts stores that offer free diagnostics. They will be able to access the codes you can't get.

Waynerd

Cycling the key in an 01 will not show any codes. If you’re check gauges light is on it’s because the key is in the run position and the engine is not running.

I have had XJ’s
88 w/250,000 first one 17 years ago est.
98 w/219,000 currently
98 w/167,000 currently
99 w/105,000 or so on the motor currently
01 w/220,000 sold a year ago
A ZJ
96 can’t remember the mileage over 150 but under 200
maybe 8-10 years ago I last drove it.
A WJ
02 w/170,000 got totaled maybe 5 years ago

all with the 4.0. They all had/have good oil pressure. Low to mid 40’s cold start and they don’t drop much until a good 10 minutes of idling after reaching operating temp 210? Only verified by the factory temp gauge. High 20’s to low 30’s hot. Using wix/NAPA filter, 10-30.

Waynerd

100C is 212F

Waynerd

And the only vehicle I can verify with photos is the 01. Keep in mind depending on how you are looking at the gauge it reads differently. I took a pic as straight on as I could which even then will not be perfectly straight. And notice from the driver seat point of view it looks to be reading higher. Pics are: cold start, when it first reaches 210 and after about 10 minutes of idling at temp.

Waynerd

1

Waynerd

2

Waynerd

3

Waynerd

This last one was after I let it idle for like 30 minutes because I knew it wouldn’t get any hotter temp or lower pressure than that. And I took it as straight on as I could.

Waynerd

I wasn’t sure if this was sarcasm at first.

Waynerd

I have heard this many times on automotive forums including this one. I have always assumed it was true but possibly they just read it on Wikipedia. Lol.

dave1123

According to Pascal's Principle, fluid in an enclosed container exerts the same pressure throughout the container. However an oiling system has many points in the system where leakage occurs that change the pressure. As parts expand due to heat, this pressure leakage increases. This also doesn't take oil viscosity changes due to temperature into account. The pressure relief valve in a Melling oil pump is set at 75 psi so it will never reach that point unless it's output is totally blocked. Increased rpm increases flow volume which overcomes pressure drop at leakage points. The question is, where is the worst leakage point when flow is low? My thought is the lifters because they are valved at a given rate regardless of engine load demands. There are also ports in the rod bearings that spray oil on the cylinder walls. What makes the AMC/jeep 4.0L more susceptible than many other engine to pressure drop remains a mystery. Jeep states the 13 psi at idle is sufficient to keep an operating film of oil on the bearings, but that's with no load. You'll need quite a bit more when the engine starts to generate power. Luckily, my 4.0 holds 35 psi at 1000 rpm in gear straining against the brakes when at 215* F. That's good enough for me.

97grand4.0

Didn't Pascale have a repair shop on the North Side? Or was it pizza?
Now I remember, it IS pizza, on the west side.

Ralph77

Of course it was. LOL.
As stated earlier I am very happy with 22 psi in an '00 with the 0331 head.
And I know for a fact it is not been replaced with a TUPY head.
But don't know if it is original to the Jeep. I am the third owner.
As matter fact I have been able to pick up a couple TUPY heads in my travels.
One from a '04 WJ for $50 and one from a '04 TJ, I think it was '04, for free.
WJ had a broken piston top. The TJ had a lower end knock. Guy who gave it to me thinks it was a rod knock.
They could possibly be used as is, could be rebuildable, could be door stops.
If and when the time comes will have them checked and go from there.

97grand4.0

Bumping this. Remember as the OP here I ask, is there a difference in the xj that makes the oil pressure worse, or seem worse. So far it looks like xj's run a bit hotter, Mine runs around 205F. 155,000 mi. Oil pressure starts over 40 and dips to 18. My zj holds around 194. 212,000 mi, Oil pressure starts over 40 and dips to 33 ish.
As Waynerd points out, water boils at 212. Pretty close.
So, when oil approaches the boiling point of water, does it do anything special.. like thin out?
My oil pan on my xj is shabby and leaks alot. So am wondering if I might attempt replacing it and replacing the rod bearings to help the oil pressure.
That is, if I can't cure it by making it run cooler.
*** UPDATE Did I answer my own question? 205F? I just checked rockauto and the xj takes the same T stat as the zj, 195F. So, I have never changed the T state on the xj. Given that it takes a 195 stat, it seems to me to be not cooling properly. I had it in my head that the xj's were JUST supposed to run a bit hotter.