• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Cold Oil Pressure at 80 psi...

Status
Not open for further replies.

tn_outside2003

NAXJA Forum User
Location
Tennessee
My 4.0 is stock with 170,000 miles on it. Recently I got a salvaged 2005 Wrangler TUPY head and had it remanufactured and put on the XJ. I had the pan pulled and the oil pump replaced at the same time.

After this partial rebuild, the cold start/run oil pressure surges up to 75-80 psi under acceleration until warmed up when it settles down to about 45 under acceleration. A HOT idle is about 18-25 psi.

Oil is 5W-30.

The cold oil pressure seems a bit high to me. This is NOT a sending unit issue. Please do not post replies suggesting using a manual pressure gauge, or suggesting swapping sending units. The question is VERY straightforward and simple. It is a SINGLE QUESTION. ONE QUESTION ONLY:

Is 80 psi, under acceleration, in a cold 4.0, too high?

Answers would read something like this:
"That pressure is fine."
"That pressure it too high and something is wrong with your oil pump/pressure relief valve."
"My XJ has the same pressure."
"Your engine is fine and that pressure is normal."
"Your engine is not fine and that pressure is way too high."

Anyone?

Thanks.
 
Mine does the same thing.....385.000, I dont worry.
 
Whenever you notice some strange oil pressure readings, the simplest thing to do is to replace the sensor. They fail quite often. I always have a spare and already have replaced them several times on all of my XJs. It’s a 5-minute job.

If you have time & resources, connect a manual pressure gauge to verify the actual pressure.
 
For what it is worth...

The FSM states that a hot idle oil pressure of 12psi is acceptable. And that 40 psi cold is to be expected. There have been numerous reports of high pressure with no particularly good explanation to the phenomena. Personally, I would verify with a mechanical gauge. It looks like cheap replacement senders run in the mid $30 range.

Personally, I would not trust this unconfirmed reading for, if it is falsely high, trouble could happen and you would not know it until it was way too late.

If I remember it correctly, the Jeep engine oils the bottom end first along with the camshaft bearings. Oil is passed up to the head via the combination of the hydraulic lifters and the pushrods which are hollow. The bores that contain the lifters have oil feed holes that pressurizes the lifter. Oil bleeds from the lifter to lubricate the cam lobes as well as passing up to the head. The head itself, does not have any pressure requirements. The oil that spills out of the top of the pushrod provides the lubrication requirements of the rocker assemblies. Which is not all that much really. Be amazed at just how little is needed by the top end of a cam in block engine...

That being said, changing the head should not result, IMO, in an overall pressure change in the system.

But, I guess I could just be in the weeds....
 
If you are running a Fram filter, get rid of it. Otherwise, you are just fine.
 
Thanks for the replies.
A couple responded with useful information.

UNBELIEVABLY, most answers STILL included "check your sender" and "use a mechanical gauge" even though the OP specifically addressed this.

WOW.

Thanks to the responders who offered information other than "change your sending unit."

If I sound annoyed...it is because lately ANY question about oil pressure...ANY QUESTION AT ALL, no matter WHAT, gets multiple answers of "replace your sending unit" and "use a mechanical gauge." The responders don't even read the question or even think for more than a split second. They are like mindless robots......"I saw the words OIL PRESSURE..... MUST RESPOND with 'change sending unit' or 'use mechanical gauge.'" It has become nearly impossible to have ANY discussion regarding oil pressure because EVERY answer offers the same thoughtless, mindless, automaton answers of "change your sending unit" and "use a mechanical gauge."

I honestly think someone could post "What IS oil pressure?" and more than half of the answers would be "change your sending unit" and "use a mechanical gauge."

Rant finished.

Thanks again to the responders who actually offered an answer OTHER THAN "change your sending unit" and "use a mechanical gauge."
 
@winterbeater:
Huh?
The 4.0 has a mechanical oil pump driven by the crank. If the key is on but the engine is off...there is zero pressure because the engine is not running.

Thanks anyway.....
 
well *did* the OP verify with a mechanical gauge?

There's a reason that these are the knee jerk responses to oil pressure issues. My own sending unit failed out of the blue a few weeks ago. They are known to do this, and in ways that produce abnormal gauge readings that still look like they may be correct and indicative of a problem.

Personally I wouldn't like to see 80 PSI with a 30 weight oil myself, if that's a true reading. It's more than the bearings need, will sap horsepower driving the pump, and theoretically could cause bearing erosion although whether that will actually happen or not I don't know.
 
80 psi is a perfectly acceptable cold oil pressure. It also happens to be the relief spring pressure in the oil pump.

Just means you have a well functioning oil pump and a tight engine. OR a failing sending unit. Use a mechanical gauge to verify the dash gauge as they are known inaccurate and are more of a suggestion as to the oil pressure that a true value.
 
Last edited:
@N8N:
Thanks for another knee-jerk response.

It was outright stated, in the original post, that I have several sending units laying around and I swapped them out and got the same readings.

"My head hurts."

"Did you use a mechanical gauge?"

Jesus.....

@Digger: THANK YOU.
 
well.. you wanted an honest answer. XJ dashes and sensing unit are well known for failing and or being inaccurate. 80 psi is not common on the 4.0 when cold.
 
@N8N:
Thanks for another knee-jerk response.

It was outright stated, in the original post, that I have several sending units laying around and I swapped them out and got the same readings.

If it was, then you edited it out, because it doesn't say that. It just says "it's not a sending unit issue." How do we know that you 100% know that?

"My head hurts."

"Did you use a mechanical gauge?"

Jesus.....

Well, did you? It's a legitimate question and if you want real help tell us what exactly you did and didn't do, don't go all "don't tell me the obvious answer" without explicitly stating that you've ruled it out by testing properly. Obvious answer is obvious because it's the most likely! *facepalm*

I'm guilty of it too sometimes, posting things like "I know it's not 'x'" without posting my troubleshooting methods, but I don't get all butthurt when someone responds with the proper method, I just say "I did that already, sorry for not mentioning that" and move on.

And the OPRV doesn't open at 80, at least not on mine... when I have a mechanical gauge with no damping hooked up I can see it open and close around 60 (at the sender port at least) when I rev the engine. Oddly my test gauge reads about 5 PSI higher than the dash gauge, not sure why but it doesn't bother me enough to worry about it.
 
The OP had the oil pump replaced... aftermarket pumps usually relief @ 80.

Wonder why? Doesn't seem particularly desirable for a (relatively) low RPM engine like a 4.0. Does the 4.0 like/want higher oil pressure than the old 10 PSI/1000 RPM rule of thumb would imply? Or is this just the aftermarket not getting it quite right?

Anyway, I'll file that in the Today I Learned file, no experience with aftermarket Jeep engine internals.
 
Status
Not open for further replies.
Back
Top