Oil Pressure reading

Cottontail

Three-De Off-Road
NAXJA Member
I just got a new XJ 2 days ago - 2000 Limited.

The oil pressure was reading low when I got it, so I put in a new oil pressure sending unit and it continued to read low. I did a complete oil change on it today. Six quarts and a new oil filter.

It continued to read low so I swapped out a cluster that has a know good oil pressure gauge, in order to eliminate that as a factor. That cluster also read low.

I drove it 40 miles or so today. The OPG shows about 20 under load on the highway, backs down to about 10 at speed if I am not on throttle, and bottoms out at zero at idle - triggering the "Check Engine Light."

It has some oil leaks - it is a Jeep after all - but there is absolutely zero noise coming from anywhere in the engine that would indicate a bad pump.

Thoughts?
 

Tim_MN

Freakish Hand Strength
NAXJA Member
Genuine Jeep part, or some crappy Chinese clone part ? Confirm the gauge indications with a mechanical oil pressure gauge. Does this 2000 have the 0331 head, and is it cracked ?
 
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Cottontail

Three-De Off-Road
NAXJA Member
I took it to my mechanic and he put the Snap-on manual gauge on it and it is indeed reading 5 psi at idle.

The truck sat 8 months. The mechanic wondered if maybe sludge settled and is blocking the oil intake tube?

I may have them drop the pan, clean it out, and see if that makes a difference.
 

tjmotter

NAXJA Member
NAXJA Member
I took it to my mechanic and he put the Snap-on manual gauge on it and it is indeed reading 5 psi at idle.

The truck sat 8 months. The mechanic wondered if maybe sludge settled and is blocking the oil intake tube?

I may have them drop the pan, clean it out, and see if that makes a difference.

If they go that far have them replace the oil pump with a Melling High Volume (NOT High Pressure). The High volume pump is great for a high mileage engine since it pushes a lot more oil through the journals. My '90XJ had 180,000 miles when I bought it and it showed low pressure at idle (not as low as yours but lower than I would like). While changing out the rear main seal (another easy job once the pan is off), I put a new High Volume pump in and my pressure now sits steady at close to 40. I drove it last week when the outside temp was over 100 degrees and the oil pressure never dropped.
 

Cottontail

Three-De Off-Road
NAXJA Member
Well, didn't do anything to the Jeep, and the last two days that I've started it and let it idle, it's held pressure at 40 on the gauge.
 

tech1001

New member
Your oil pressure will be higher when the engine is cold after driving and the oil is up to temp then it will be lower . You have a 2000 xj that the head will crack and put coolant in to the oil and your oil pressure will drop , You can see if you are leaking coolant into the valve cover , if you pressurize the cooling system then take off the oil cap and look inside between the valve springs at the top of the head if your head is cracked you can see the coolant coming though the head , or if you see the word tupy that is a newer head that doesn't crack , there are other heads that can replace the head you have , you will also be losing some coolant with no leaks to be seen .
 

Cottontail

Three-De Off-Road
NAXJA Member
I just drove it for awhile, and yes, it held steady at 40 but dropped at the temp gauge got closer to 210. By the end of the trip, I was back to the gauge showing zero and throwing the check light at idle.

Need to check the head still.
 

Cottontail

Three-De Off-Road
NAXJA Member
I don't know what to believe anymore.

I've got people saying that my engine is about to blow up or is all but destroyed.

I've got others saying that 1 psi per 100 rpms is OEM spec - so 7 psi at 700 idle rpms is fine.

I've got others saying the FSM shows 13 psi at idle is minimum.

At this point, I'll just clean it all out, put in a new pump, and drive it until it blows up then swap in the motor from the wrecked XJ, I guess.
 

asymptonic

NAXJA Member
NAXJA Member
I don't know what to believe anymore.

I've got people saying that my engine is about to blow up or is all but destroyed.

I've got others saying that 1 psi per 100 rpms is OEM spec - so 7 psi at 700 idle rpms is fine.

I've got others saying the FSM shows 13 psi at idle is minimum.

At this point, I'll just clean it all out, put in a new pump, and drive it until it blows up then swap in the motor from the wrecked XJ, I guess.

No, this I can say definitively, from the 4.0 engine section:


"The MINIMUM oil pump pressure is 89.6 kPa (13 psi) at 600 rpm. The NORMAL oil pump pressure is 519 kPa (75 psi) at 1600 rpm or more"


7psi is right out.


But make sure this is on a mechanical gauge. The electric dash gauge is a nice approximation if working well but can definitely be off.


The reason folks are telling you you have shot bearings is that as the bearings wear, they allow oil to move more freely through the engine and thus reduce pressure. It's not that people are guessing that your bearings are shot, it's just that it's the most likely symptom of low oil pressure.
 

md21722

NAXJA Member
NAXJA Member
"The MINIMUM oil pump pressure is 89.6 kPa (13 psi) at 600 rpm. The NORMAL oil pump pressure is 519 kPa (75 psi) at 1600 rpm or more"

This isn't quite right - it should be 37-75 PSI at 1600 rpm or more
 

asymptonic

NAXJA Member
NAXJA Member
"The MINIMUM oil pump pressure is 89.6 kPa (13 psi) at 600 rpm. The NORMAL oil pump pressure is 519 kPa (75 psi) at 1600 rpm or more"

This isn't quite right - it should be 37-75 PSI at 1600 rpm or more

That's the FSM text unless I missed a spot, but I agree, I think they meant to say max PSI at 1600 rpm is 75.
 

tech1001

New member
Try a 10w30 or 10w40 a lot of people run 15w40 and see how your oil pressure is . a 5w30 is on the light side for the jeep .
 

lawsoncl

New member
Try a 10w30 or 10w40 a lot of people run 15w40 and see how your oil pressure is . a 5w30 is on the light side for the jeep .

I agree, try a 10w-40. A 5w-30 is the same viscosity as a 10w-40 once the engine is hot. Run a decent filter too, btw. No cheap fram junk. I usually go with Wix/Napa silver myself. The pressure sensor is downstream of the filter, so a poor flowing filter will lower the measured pressure.
 

Cottontail

Three-De Off-Road
NAXJA Member
I only have about 30 miles on this oil change, but I'm pulling the pan next weekend to change to a high volume oil pump. I have a Mopar filter on it now. I'll just leave that one in place and refill with a 10-40 next week.

Going to do the RMS at the same time.

If that doesn't do anything I'll pull the check gauges light out of the dash so it doesn't bother me and run it until she blows. It's whisper quiet right now. I have another engine ready to put in it, if need be.
 

tech1001

New member
If you go and some of threads on oil you find out that most use a 10w40 or 15w40 and the oil pressure does go to about 15-20 psi at idle when hot .
 
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