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Unread 11-01-2010, 06:46 AM   #1
xjman4life
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01 XJ 170,XXX oil pressure drops to zero

My DD XJ that I drive 100 miles a day started up with a new problem this morning. When I was stopped at a light the oil pressure slowly dropped to zero and the check guages light came on. When I put it in nuetral and revved it the pressure climbed back up. Once I am driving it goes back to normal. It only happens when idling. What is going on? My 89 did this b4 and it was the oil pressure unit by the oil filter. Does the 01 have the same thing cause it sure looks different so I wasn't sure. I need this thing running good cause I DD it 100 miles a day. Help please.

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Unread 11-01-2010, 07:07 AM   #2
tjwalker
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It could be a faulty oil pressure sending unit, located right above the oil filter.

Does the engine make more noise when the pressure goes to zero?? If your pressure truly was near zero, it should start making some noise.

The best strategy would be to get a hold of a mechanical oil pressure gauge (can be rented from parts stores) and temporarily install it where the oil pressure sending unit resides. That way, you'll know for SURE.

Or you could just try a new sending unit. I buy mine directly from Jeep for best reliability/accuracy/mating with your factory gauge.

Replacing the oil filter wouldn't be a bad idea, but that is more of an outside chance.

Low pressure readings always concern me and I would resolve/verify this ASAP.
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Unread 11-01-2010, 07:33 AM   #3
xjman4life
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yeah, I definitely want to get this resolved. I am pretty sure its the pressure unit. I will pick one up after work today. I just hope I can get to the store. Its reading fine when its at driving speeds..just not at idle. I think it is reading incorrectly. If it was making noise I doubt I would hear it since I do not have perfect hearing.
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Unread 11-02-2010, 08:29 AM   #4
hollandbr2
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Possible stuck pressure relief valve

I would be careful with that one. Mine did the same thing one day, when I rev'd it up it would come up to 20'ish and then fall back down at idle. I put a new pressure sending unit (There are two of them one for the dummy light and one for the gauge make sure you get the right one). It started reading a little bit higher reading; 30ish and when it fell it would be 10-15ish. Kept driving it and for the most part it went away.....then the loud top end rattle started

Long story short after pulling the oil pump there was some black sludgy grime in the the spring/pressure relief valve on the oil pump. The valve would not close all the way and I did a excellent job of recirculating oil in the pan. Cleaned it with some brake cleaner and put it back on, saved my engine.

My friends started the same thing, I told him about it, he replaced the sending unit, drove....then the loud bottom end knock started. Pulled his, same sludgy grime built up in the spring but the damage was already done to his crank.

Its an easy job, drop oil pan, three bolts on the pump and its down, roll pin, clean, reassemble. Starting with the sending unit is a good idea but be careful cause you really want to avoid this: Jeep Cherokee Stroker Engine - Discarded Engine Parts from 4.7L Stroker Build-Up.
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Unread 11-02-2010, 09:36 AM   #5
NYJeep2000
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Thanks. I have the same problem with dropping at or near zero when the oil is hot and I am idling at a traffic light. It climbs back up just like described above. I have already replaced the sending unit with no help. When it is cold there is plenty of pressure. I was thinking maybe oil pump but now for sure I will be dropping the pan.
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Unread 11-02-2010, 04:02 PM   #6
hollandbr2
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If you replaced the sending unit and it is still low then you need to park it. The oil pump is $70'ish at my local auto parts stores and $60 at amazon here:
Amazon.com: Melling M81A Replacement Oil Pump: Automotive: Reviews, Prices & more

It is truly a fairly simple job. Mine has a life so I didn't even have to jack it up, but my friends didn't and jacking it up helped to allow more clearance in removing the pan. Jack it up, drain oil, remove the pan bolts (the four corners are different size, take note for putting them back in), drop the pan, remove the one bolt from the pump pick up tube, remove the tube, remove the three bolts from the pump, remove the pump.

Once you have the pump down you can take it apart, measure the clearances with shims, take out the cotter pin, remove the spring and plunger for the relief valve, clean all up and reassemble. Or, just replace with a new one.

Then just reassemble. You really do not want to fight with, and/or deal with the leaks afterwards, buying the 4 piece oil pan gasket. Just spend extra $5 for the one piece and you'll thank yourself, its only $21 here for a Felpro at my local parts store $24 at amazon here:
Amazon.com: Fel-Pro OS34308R Oil Pan Gasket Set: Automotive: Reviews, Prices & more

There is the possibility that you have some sort of internal leak, or bearing clearances, etc. are all so worn that you can't keep oil pressure and that is why its so low, bla bla bla.....but no. My experienced suggestion is your oil pump is worn and/or gunky with tar and other black particles that is keeping your pressure relief valve open.

I actually have a spare pump from an engine I bought for another project. It came with one and I purchased one online. However, I would want 20-30 for it plus shipping you might as well just buy one. If you have an amazon prime account second day shipping is free anyway.

Again, I HIGHLY recommend you heed to this gauges warning and park it till you fix it!
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Unread 11-24-2010, 04:59 PM   #7
Higgs98XJ
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Just chiming in to 2nd the "park it" recommendations made in this thread. I've been driving my dad's WJ for the past week and it was doing this. Seeing as how it isn't my car and i've been strapped for cash I decided to risk it and ignore the warnings. Stubborn mistake.

Most of my driving takes place on a highway anyway so I didnt have to worry about problems at idle.

BUT

While traveling down the highway I noticed I was overheating and IMMEDIATELY pulled over to have it towed. It was running smooth while this all happened (no seize). The music was up, so I probably wouldn't have noticed any knock. Popped the hood to let it cool and noticed that my coolant was low but not low enough to be the sole cause of overheating. I filled the coolant a week ago.

IF YOU ARE READING LOW OIL PRESSURE, PARK IT! and for god's sake don't be cheap and refuse to drop $50 on a new oil pressure send unit. You could end up spending a lot more money for making the same mistake I did.
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Unread 11-24-2010, 08:23 PM   #8
jlimit1
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If you must drive it at the very least put some Lucas oil stabilizer in there. When mine did the same thing it brought the pressure to safe levels, especially at idle.
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Unread 11-24-2010, 09:38 PM   #9
xjman4life
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Sorry for the lack of updates, I ended up fixing it the day it happened. I drove to Advance right after work less than a mile away, picked up the part and stood on my tire and reached down into my engine bay. I had to avoid getting my work clothes dirty on the side of my jeep so I stood on the tire. The oil pressure is holding fine, turns out the pressure sensor had come apart at the seam where the metal and plastic meet. It was spraying oil everywhere and it would spin as two separate pieces instead of one sealed unit like the new one I picked up. It was a little pricey for the part, almost 60.00 if I remember correct. Awesome fix if I might say so myself, shes still running strong. Did a spark plug replacement and an oil change the other day.
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Unread 11-25-2010, 07:44 AM   #10
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by xjman4life View Post
The oil pressure is holding fine, turns out the pressure sensor had come apart at the seam where the metal and plastic meet. It was spraying oil everywhere and it would spin as two separate pieces instead of one sealed unit like the new one I picked up:
You are lucky you got it replaced before the engine ran low on oil and
created some real problems.

Glad you got it fixed.
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Unread 11-27-2010, 06:19 PM   #11
mx2times
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While we are on the subject, what would be normal operating oil pressure ranges?
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Unread 11-27-2010, 06:33 PM   #12
tjwalker
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Quote:
Originally Posted by mx2times View Post
While we are on the subject, what would be normal operating oil pressure ranges?
Oil pressure spec. for the Jeep 4.0 is as follows:

*Not less than 13 psi at hot idle
*Between 37-75 psi above 1600 rpms.

Sidebar: I have seen more than a few XJs with the 4.0 run for a long, long time on about 10 psi verified pressure.
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Unread 11-27-2010, 06:58 PM   #13
mx2times
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Thank you!

Our 2000 at hot idle runs right at about 10 psi. She has 155,000 and we run 5-30. Are we okay, or should I consider a thicker viscosity?
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Unread 11-27-2010, 07:44 PM   #14
tjwalker
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Quote:
Originally Posted by mx2times View Post
Thank you!

Our 2000 at hot idle runs right at about 10 psi. She has 155,000 and we run 5-30. Are we okay, or should I consider a thicker viscosity?
What I would do if I saw my oil pressure at 10 psi on the dash gauge would beto verify that with a "mechanical" oil pressure gauge.

They can be rented at many big parts stores for a few bucks. Remove the oil pressure sending unit and screw in this mechanical gauge.

Then you will know for SURE what oil pressure you are running at.

The reason for this is that there can be some inaccuracy on your gauge.

Good data = good decisions.
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Unread 11-28-2010, 12:39 PM   #15
AZ Jeff
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Quote:
Originally Posted by tjwalker View Post
What I would do if I saw my oil pressure at 10 psi on the dash gauge would beto verify that with a "mechanical" oil pressure gauge.

They can be rented at many big parts stores for a few bucks. Remove the oil pressure sending unit and screw in this mechanical gauge.

Then you will know for SURE what oil pressure you are running at.

The reason for this is that there can be some inaccuracy on your gauge.

Good data = good decisions.
I will also chime in and say that 5W30 is a bit light for an engine of the type the 4.0 is. That engine was designed by AMC engineers when nobody ran anything lighter than a 10 weight oil, and thus the oiling system is sized accordingly.

I would run nothing lighter than 10W30 in a 4.0 engine, except in REALLY cold climates like northern Canada or Alaska.
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