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Oil pressure drop to 0 at idle when engine warms up.

Old 04-21-2019, 09:09 PM
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Default Oil pressure drop to 0 at idle when engine warms up.

Hey guys. Starting a few days ago, my 96 Cherokee XJ 4.0 would startup cold reading about 40psi on the oil gauge, but drop all the way down to 0 after it was warm (at idle). I decided to replace the sending unit and got the same results. So, I then bought a cheap mechanical gauge and hooked it up to a t-fitting so I could see if the gauge was reading correctly. Same results. This time I decided to hit the gas and raise the rpms a bit to see if the pressure increased at all. At 1250rpms, I'm showing a steady 38psi. I'm not noticing any loud tapping/knocking when it's at 0, but I'm not letting it run long in the driveway at 0 for that to happen either. I attached some pics below of idle and with a little gas. If it were the bearings, I would've thought the pressure would gradually have lowered over time, but this just suddenly changed the other day from running close to 40psi to 0. At 110000 miles (if true), it seems low for the engine to be that worn or for a pump to be going out. But, if it were either, I was hoping it's the pump.

To top it off, I'm moving in a week and working on this in a pea-gravel driveway. (ugh). My next step was to drop the oil pan tomorrow, but wanted to pick your brains before tearing into it too deeply. Thanks for any ideas.


After engine is warm.

Giving it some gas.
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Old 04-21-2019, 10:52 PM
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Instead of dropping the pan initially I think I would drain the oil and examine it for metal glitter. If nothing alarming then change the filter, refill the oil and see what you have. Seem like an easier start than removing the pan, working in gravel. I would not run the engine for any duration while showing very low oil pressure until you get this figured out.
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Old 04-21-2019, 11:07 PM
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I don't remember what years have what for the oil pressure sender, but I know some have a little 90 degree adapter that you could unscrew and make sure it's not clogging or anything
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Old 04-22-2019, 02:34 AM
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If this were an '00 or '01 and because my mind immediate goes to the bad place I would think cracked head. But being a '96.......
Thinking do what third coast and SatiricalHen said first.
Any unexplained loss of anti-freeze?
Then I am wondering what tests can we run to try and narrow this down.
After owning my '00 for about 5 years I got it in my head that I wanted to see what condition my engine was in.
So I bought all the tools I needed to run tests. Frankly was just looking for an excuse to buy tools. LOL.
Bought a compression tester, leak down tester, block tester, coolant pressure tester, and a mechanical oil pressure gauge tester.
So you have already put a mechanical gauge to the engine.
So now somebody smarter then me is going to have to chime in here.
But will any of these tests help determine why?

And it might be a little late in the game cause you are moving but I have the same problem with my driveway.
Picked up a short bed truck mat for $20 off Craigslist and it fits perfectly under the Jeep. Tires are not on it so you can move it some.
Picked up this 3'X4' mat at Tractor Supply on sale for $20 and cut it into 3 pieces. One 1.5'X4' and 2 1.5'X2' pieces.
the 1.5'X2' ones are perfect for sitting or kneeling and the 1.5'X4' one is perfect to lie on around the outer edges of the Jeep
https://www.tractorsupply.com/tsc/pr...t-x--frac12-in

Last edited by Ralph77; 04-22-2019 at 02:50 AM.
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Old 04-22-2019, 04:25 AM
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Originally Posted by betic72 View Post
Starting a few days ago, my 96 Cherokee XJ 4.0 would startup cold reading about 40psi on the oil gauge, but drop all the way down to 0 after it was warm (at idle).
Did you purge the oil sending unit port before you put the new one in?

https://www.jeepforum.com/forum/f19/...fixed-1304679/

Last week my 2002 WJ 4.0 4x4, 140,000 miles, started having low oil pressure at idle. I was at a stop light and the "check gauges" light came on. I pulled over and checked my oil level and it was good. The oil pressure would rise with rpms but would go back to almost zero at idle. I let it cool off and noticed that pressure was normal on cold start but would slowly drop over a few minutes as the engine warmed up. I searched here and on the web and found lots of info. I first tried a replacement sender but there was no change. I cleaned the connector with electrical cleaner since there was some oil around the pins - no change. I dug around and found an old mechanical oil pressure gauge and installed it and left it hanging in the engine compartment. Showed same as the dash gauge - low pressure, about 8-10 psi at idle and rising to 55-60 at 2500 rpm. I let it idle for a while but heard no odd noises. Just the same as before - good pressure on cold start and decreasing as engine warms. Went back and searched some more and found info about the oil gallery behind the sending unit getting clogged with sludge. So I took off the mechanical gauge and left the brass adapter in the hole and stuck a short piece of rubber hose to that and into a drain pan. I then turned it on and revved it at 4000 for about 8 seconds and shut it off. I checked the drain pan and found some obvious bits of sludge that had been blown out. I connected the mech gauge again and topped off the oil. Turned it back on and problem solved! Pressure at idle now 18-20 psi and 60 psi at 2500. Replaced original sender and dash gauge was back to normal. Hope this helps someone with the same problem.
Pull the oil filter and dissect it to make sure the filter material hasn't collapsed. If that don't help take apart the oil filter adapter and make sure that's all clean, especially part #21:


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Old 04-22-2019, 07:29 AM
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I haven't purged the sending unit port. I ran across some posts about that last night. That's a good idea and probably the thing I'll try next. When I pick up some oil this morning, I'll grab a rubber hose for the port then give the oil filter another look after that. The oil filter adapter was apart yesterday (changed the o-rings on those for both Jeeps) and it didn't have any sludge in it. I didn't take a close look inside of the oil filter when I had it off so I'll look at that again. I'm changing the oil, but waiting to see if I have to drop the pan before doing that.

Good idea about the mats under the Jeep. I have a mostly intact piece of plywood I can slide under there for now that'll serve the same purpose, but definitely not as easy to move around. I'll report back my findings on the purging this morning. That would be great if it was the cause.
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Old 04-22-2019, 07:45 AM
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Originally Posted by betic72 View Post
Good idea about the mats under the Jeep. I have a mostly intact piece of plywood I can slide under there for now that'll serve the same purpose, but definitely not as easy to move around.
I leave the truck mat out during the warmer months and bring it in for the winter. Just cause when the landlord uses the snowblower on the driveway I don't know if it would cause a problem.
If I knew for sure that it would not I would buy bigger Tractor Supply ones and set it up so my Jeep would fit on it and extend out all around it.
Thing about the Tractor Supply mat is that it is thick and can't be rolled to be stored.
The truck bed mat can be.
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Old 04-22-2019, 07:53 AM
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I use old carpet. When it gets really bad I toss it and get another piece.
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Old 04-22-2019, 11:54 AM
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Well, I purged the sending unit port and looked at the old oil filter. Filter was still in good condition although oil is dark but is getting changed after this process. I topped off the oil that I lost and the engine got warm and ran for about 5 minutes at 210 before the pressure slowly started to drop until finally it hung out a little below or at 10psi and I had to shut it off. Engine isn't making any odd noises and is running smoothly when it's on. Any other ideas before it gets to main bearings or pump? I don't see any presence of coolant in the oil at all. Not watered down, just dark. So a cracked head probably isn't a factor.
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Old 04-22-2019, 12:33 PM
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Originally Posted by betic72 View Post
...it hung out a little below or at 10psi and I had to shut it off. Engine isn't making any odd noises and is running smoothly when it's on.
"Normal" oil pressure for 4.0 at 600 RPM hot is 13 psi.

What weight oil is in there now?

IMO the important point is that this happened suddenly.

A malfunction of the pressure relief valve?
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Old 04-22-2019, 12:37 PM
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Originally Posted by Dave51 View Post
"Normal" oil pressure for 4.0 at 600 RPM hot is 13 psi.

What weight oil is in there now?

IMO the important point is that this happened suddenly.

A malfunction of the pressure relief valve?
Pretty sure that technically 13 PSI is the low end of still OK.
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Old 04-22-2019, 12:39 PM
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Roger Ralph.
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Old 04-22-2019, 12:43 PM
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I lied...

But still, 3 crummy pounds...

Last edited by Dave51; 04-22-2019 at 12:49 PM.
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Old 04-22-2019, 12:46 PM
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Originally Posted by Dave51 View Post
Roger Ralph.
With an '00 you worry more about this. Was happy in my ignorance just relying on my
gauge in the cluster. About 22 PSI at a hot idle. Finally broke down and checked it.
Set it up so I could run the mechanical gauge and the sensor at the same time.
Was both happy and very surprised with the results.
Very hot idle.

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Old 04-22-2019, 01:03 PM
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I did read where "they" said "not less than 13 psi", but 2000 FSM just says plain old 13 psi. It's so close, and I'm sure a new oil pump would add plenty of of lbs., and the purge seemed to help...

Could try thoughtful prayer...

Like "Dear Lord could you put oil pumps on sale this week?"
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