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post #1 of 9 Old 01-22-2012, 04:56 PM Thread Starter
michaelleegarre
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2000 TJ Wrangler 
 
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oil pressure problem

I have a 98 Cherokee with 137000 miles when i first start it i have good oil pressure around 40 driving down the road or when im sitting but after awhile when the engine worms up it drops to around 10 sitting but when i take off it goes back to 40 what could this be


Michael Garrett
1987 YJ - sold, 1995 ZJ - sold, 1998 XJ - sold, 2000 TJ
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post #2 of 9 Old 01-22-2012, 05:07 PM
tjwalker
It's the crank sensor!
 
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Location: Central Minnesota
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Oil pressure fluctuation is normal with your XJ.

Here is the oil pressure spec for the Jeep 4.0 engine:

*Not less than 13 psi at HOT idle

*Between 37-75 psi over 1600 rpms


If you are in the above bracket, you are fine. If you are not, you should consider testing with a "mechanical" gauge, which can be rented from many big box parts stores. Also know that there can be some inaccuracy between the oil pressure sending unit and the dash gauge, usually caused by a faulty sending unit. If you buy one, buy it directly from Jeep for the best reliability/accuracy/mating with your factory dash gauge.

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #3 of 9 Old 01-22-2012, 05:14 PM Thread Starter
michaelleegarre
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i was thinking that if i wasn't getting oil the engine would be making noise

Michael Garrett
1987 YJ - sold, 1995 ZJ - sold, 1998 XJ - sold, 2000 TJ
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post #4 of 9 Old 01-22-2012, 05:25 PM
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by michaelleegarre View Post
i was thinking that if i wasn't getting oil the engine would be making noise
Oil pressure often has to get pretty low before you start hearing engine noise

Verify if there is any question. Then you'll know.

Good data = good decision. No guesswork.

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #5 of 9 Old 01-22-2012, 05:28 PM Thread Starter
michaelleegarre
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thanks for the help im not freaking out now thinking im about to blow up my engine

Michael Garrett
1987 YJ - sold, 1995 ZJ - sold, 1998 XJ - sold, 2000 TJ
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post #6 of 9 Old 01-22-2012, 05:59 PM
redcar
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I also ran into weird oil pressure readings (super low, then back to norm, then low again, etc.) turned out to be a bad oil pressure sensor.
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post #7 of 9 Old 01-22-2012, 07:06 PM
Bill-Horn
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This oil-pressure thing is subtle. Oil pressure is variable with engine temperature, engine age, oil specification, oil age, oil-change frequency, oil additive, ambient temperature, bearing condition, sensor accuracy and your zip code, birth date and shoe size. You think I'm kidding about the last two, but I'm not. Your driving style affects oil pressure.
The analog pressure gauge gives you lots of information about the general condition of your engine; how it's performing under load and if something serious is happening (or about to happen) with the crankshaft bearings.
If you divide the total square-inch area of the crank bearings by the indicated oil pressure, you get a picture of how well the motor is lubricated under load. Ideally, there should be absolutely no friction between the crank and the connecting rods, thus the perfect lubricant: Liquid Rock (petroleum). As wear-and-tear gradually increase the space between the moving parts of your engine, absolute oil pressure decreases. But it does not decrease absolutely; temperature affects viscosity and suspended solids and dissolved byproducts of combustion affect the capability of your oil to lubricate.
Change your oil frequently. Acid buildup deteriorates your engine's bearing surfaces and carbon abrades those polished surfaces your bearings need to slip and slide smoothly and silently. Change your oil frequently. The marketing machines say 4000-miles...I say twice a month regardless..like every other tank of gas. I'm not here to sell oil, but my '89 Cherokee 4.0 6-inline runs better every year and is a legacy item for my grandson.
Every once-in-a-while, unscrew the pressure sender and start the motor for about five seconds. What will happen is that oil will blast out of the motor all over your garage floor, but all the crud and debris in the oil pan will come with it. Get clever and you can do it without making a mess.
If the pressure gauge is flinging itself from left-to-right with wild abandon, it's probably an electrical problem. The wiring in the Jeep is fragile; connectors come unseated, insulation wears, wires break, etc. Troubleshooting electrics is a *****, but it's good to learn how to do it because your local mechanic can't invest the time to do it for you and make a profit on the work involved. Get-down!, bro..and learn.
The instrument cluster seldom (if ever) fails. It's just too basic. If your oil pressure is fluctuating and you're getting nervous, it's either the sender, the wiring, or dirt/debris in the veins and arteries of your motor. Marvel Mystery Oil is an excellent medication for clogged oil passages, but be prepared to change your oil and filter as you apply the cure because all the stuff clogging-up the system will come loose and has to be removed.
Change your oil.
Change your oil.
Do it again.
Cheers=Bill

"Everyone is entitled to his own opinion, but not to his own facts."
1989 Cherokee XJ
250,000 Miles
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post #8 of 9 Old 01-22-2012, 07:35 PM
Chrisp929
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Quote:
Originally Posted by Bill-Horn View Post
This oil-pressure thing is subtle. Oil pressure is variable with engine temperature, engine age, oil specification, oil age, oil-change frequency, oil additive, ambient temperature, bearing condition, sensor accuracy and your zip code, birth date and shoe size. You think I'm kidding about the last two, but I'm not. Your driving style affects oil pressure.
The analog pressure gauge gives you lots of information about the general condition of your engine; how it's performing under load and if something serious is happening (or about to happen) with the crankshaft bearings.
If you divide the total square-inch area of the crank bearings by the indicated oil pressure, you get a picture of how well the motor is lubricated under load. Ideally, there should be absolutely no friction between the crank and the connecting rods, thus the perfect lubricant: Liquid Rock (petroleum). As wear-and-tear gradually increase the space between the moving parts of your engine, absolute oil pressure decreases. But it does not decrease absolutely; temperature affects viscosity and suspended solids and dissolved byproducts of combustion affect the capability of your oil to lubricate.
Change your oil frequently. Acid buildup deteriorates your engine's bearing surfaces and carbon abrades those polished surfaces your bearings need to slip and slide smoothly and silently. Change your oil frequently. The marketing machines say 4000-miles...I say twice a month regardless..like every other tank of gas. I'm not here to sell oil, but my '89 Cherokee 4.0 6-inline runs better every year and is a legacy item for my grandson.
Every once-in-a-while, unscrew the pressure sender and start the motor for about five seconds. What will happen is that oil will blast out of the motor all over your garage floor, but all the crud and debris in the oil pan will come with it. Get clever and you can do it without making a mess.
If the pressure gauge is flinging itself from left-to-right with wild abandon, it's probably an electrical problem. The wiring in the Jeep is fragile; connectors come unseated, insulation wears, wires break, etc. Troubleshooting electrics is a *****, but it's good to learn how to do it because your local mechanic can't invest the time to do it for you and make a profit on the work involved. Get-down!, bro..and learn.
The instrument cluster seldom (if ever) fails. It's just too basic. If your oil pressure is fluctuating and you're getting nervous, it's either the sender, the wiring, or dirt/debris in the veins and arteries of your motor. Marvel Mystery Oil is an excellent medication for clogged oil passages, but be prepared to change your oil and filter as you apply the cure because all the stuff clogging-up the system will come loose and has to be removed.
Change your oil.
Change your oil.
Do it again.
Cheers=Bill
One of the most sensible posts i have ever read. Although i didn't need it, i thank you bill for making so much sense for those that do. I vote that you win the "Perfect Post" Award of there is such a thing.
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post #9 of 9 Old 01-22-2012, 07:51 PM
Bill-Horn
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Whoa!

"Everyone is entitled to his own opinion, but not to his own facts."
1989 Cherokee XJ
250,000 Miles
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