Wife’s 99 Jeep 4.0L grand Cherokee lost oil pressure! - Ford Powerstroke Diesel Forum

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post #1 of 11 Old 12-27-2009, 12:38 PM Thread Starter
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Wife’s 99 Jeep 4.0L grand Cherokee lost oil pressure!

We were on vacation in the Smokey Mountains yesterday and just as we reached the parking lot at the top of the mountain the dinger went off and the oil pressure gauge read 0!

I figured out that anything over 1,500 rpm would give me 40 psi and less than 1,200 RPM would give me 0 psi.
It was pretty tricky driving and it seemed to get worse as time went on but I managed to keep the rpm’s and psi’s up until we got home this evening. In heavy slow traffic it would get to where it took over 3,000 rpm to hold 35 psi, at interstate speeds 2,000 psi would hold 38 psi but anything less than 1,500 gave me 0 psi. So it seems like it is somewhat temp related.
BTW, I run Mobil 1 full synthetic and the Mobil 1 filter and the current oil is clean and full and has around 4,000 miles on it. The Jeep has 215,000 and is in like new condition.

Anyone got any ideas where to start diagnosing it???
I’m thinking oil pump or pressure regulator?

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post #2 of 11 Old 12-27-2009, 03:12 PM
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Oil pickup in the pan might be dislodged? Only picks up when the pump is suckin hard at high RPM's?

I think any way you look at it you're gonna be pulling the motor to fix it man.

On another note, if her Jeep starts goin through front axles, the wheel bearings that look and sound fine are what causes it.
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post #3 of 11 Old 12-27-2009, 03:45 PM
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I am in agreement. It is something to do with the oil pump.

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post #4 of 11 Old 12-27-2009, 03:47 PM
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would pulling the engine really be nesessary? would the pan not be able to be removed... my old xj i pulled the oil pan in a matter of thirty minutes

but then again it was an 86...

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post #5 of 11 Old 12-27-2009, 04:44 PM
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My wife's grand cherokee has a 318, so I dunno how comparable motor depth is, but I doubt I could get the pan out with the motor in.

Maybe just disconnect the mounts and hoist it up a few inches but I dunno.
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post #6 of 11 Old 12-27-2009, 04:46 PM
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I have heard they sometimes have problems with bearing wearing out and not holding pressure

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post #7 of 11 Old 12-27-2009, 05:26 PM Thread Starter
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I have a complete factory service manual and it says I can swap the oil pump without pulling engine but I got to take to motor mounts loose and jack it up and take several other things loose. Sounds like about a 3 hour job.
I went and got a test gauge and a new sending unit and new pump. I plan to go out and use the test gauge to verify whether it is a gauge error or mechanical error then IF mechanical I reckon I’ll drop the pan and swap the pump and inspect the pickup tube then fill with new oil and filter and see what happens. It would not bother me at all to rebuild the engine but it has good compression and (until this weekend) had good oil pressure and no other issues. I’m thinking the pressure regulator in the oil pump is stuck open.
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post #8 of 11 Old 12-27-2009, 06:21 PM Thread Starter
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Problem solved!

Long story short, the sending unit that I had replaced about a year ago was the problem, new sending unit fixed it, she is now idling hot over 40 psi. Took about 5 minutes work, 1-1/16” wrench and $45 sensor to fix. It sure didn’t act like a bad sensor or electrical problem to me seemed more like a flow or volume problem. I wish I had of known it before I fought it for 450 miles getting it back home. But that is still a lot better than pulling an engine.

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post #9 of 11 Old 12-27-2009, 06:34 PM
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So you had oil pressure the whole time huh?

Lol, well at least it was a easy fix!
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post #10 of 11 Old 12-27-2009, 06:44 PM Thread Starter
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Yea, I’m lucky I didn’t break the tranny with all of the manual shifting and gear slamming, it sure worked the heck out of me. Doesn’t sound that tough but try keeping the RPM above 2,000 in heavy traffic like in Gatlinburg or Pigeon Forge or in an interstate jam in Chattanooga.
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