Oil Pressure Issues - Tried Everything - JeepForum.com
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post #1 of 20 Old 08-12-2016, 04:25 PM Thread Starter
NW99XJ
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Oil Pressure Issues - Tried Everything

1999 Jeep XJ
200 -some-odd thousand miles
4WD, 4.0L, Auto...etc...etc....etc...

Owned this rig for going on 8 years I think... and in that time I have swapped out the oil pressure sending unit 4+ times.
Mostly because every time I would do so... the low oil pressure reading I was seeing... was fixed.
OK...fast forward to this year.... I have done some mechanical work on it.... nothing I haven't done before... and yes... i know what i'm doing... been wrenching on vehicles for longer than some of you have been alive.
That being said.... I have swapped out the oil pump (because I DID have a lack of oil being pumped around in the valve train)
I have swapped out the sending unit with a genuine Mopar one.
I have removed and cleaned the sending unit elbow (even though it was already clean - ie; no blockage/sludge, etc...)
I have verified oil pressure using a mechanical gauge.
I have replaced the RMS, oil pan gasket, header gasket (unrelated)
I've replaced the valve cover gasket and oil filter adapter o-rings as well.
The gauge cluster has been tested to verify its functionality, and I've even swapped the ECU just for grins and giggles.
Despite actually HAVING oil pressure verified by a mechanical gauge AT the sending unit location.... despite having swapped out the pump and sending unit with Mopar replacements, and despite refreshing just about every ground from the front seats forward... I STILL show 5psi of oil pressure on the dash, no matter what. I know this is bogus....
So... since I am out of troubleshooting procedures for this, since the chilton and haynes manuals aren't worth the paper they're printed on, and since my FSM doesn't really provide much help towards this matter either... I
I turn to you... my beloved Jeep Brethren to help me find a solution that I haven't already thought of or tried.
What is YOUR thoughts on why this is happening or what is causing the low pressure reading?

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5+" of lift, 33's, OTK, Armored, Lighted, Chipped, Winched & Locked up Front.
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post #2 of 20 Old 08-12-2016, 04:51 PM
SemperFiGuy
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Have you traced the wiring? Looks like that was the only thing not mentioned.

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post #3 of 20 Old 08-12-2016, 05:19 PM
RLE
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I'm not an electrician and I would not know much about
how to verify a cluster's gauge.

Could you swap in another cluster to see if it, too, always
shows just 5psi?

Or as SemperFiGuy suggests, trace the wiring.
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post #4 of 20 Old 08-12-2016, 11:05 PM
getterdun
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I'm assuming that when you tested the oil pressure with a mechanical gauge the reading was normal.

And since you have swapped the sending unit multiple times, I think we can rule that out.

It seems that whats left is the wiring, connections, PCM, and oil pressure gauge. I would start by performing the gauges self test as shown in the video:

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post #5 of 20 Old 08-13-2016, 05:41 AM
SemperFiGuy
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I don't think the gauge test is going to work in this instance. I could be wrong but doesn't that just test the "mechanicalness" of the gauges by putting them through the motions?

I'm still going with corrosion in the sending unit wire harness somewhere.

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post #6 of 20 Old 08-14-2016, 01:35 PM
rjbruzan
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Does the check gauges light come on?

Ron

Of all the things Ive lost
The one I miss most is my mind

Its like a fart in a windstorm
It smells good but thats about it

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post #7 of 20 Old 08-15-2016, 10:49 AM Thread Starter
NW99XJ
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Quote:
Originally Posted by SemperFiGuy View Post
Have you traced the wiring? Looks like that was the only thing not mentioned.
Not the entire length from the sending unit to the ECU... but a good chunk of the first foot or so.... just to see if something was pinched, melted, etc...
There is a collection of grounds at a connection point just to the side of the bolt that holds the oil dip stick tube. Those were checked, refreshed and re-installed, even though they showed good continuity. Doesn't hurt for a little piece of mind i guess.

Quote:
Originally Posted by RLE View Post
I'm not an electrician and I would not know much about how to verify a cluster's gauge.
Could you swap in another cluster to see if it, too, always
shows just 5psi?
I've thought about that... just as an additional step to further eliminate the possibility of a bad cluster.... but I "THOUGHT" that the test procedure covered the functionality of it. I have enough XJ friends, that sourcing a cluster to swap real quick shouldn't be an issue.

Quote:
Originally Posted by getterdun View Post
I'm assuming that when you tested the oil pressure with a mechanical gauge the reading was normal.
And since you have swapped the sending unit multiple times, I think we can rule that out. It seems that whats left is the wiring, connections, PCM, and oil pressure gauge. I would start by performing the gauges self test as shown in the video: https://www.youtube.com/watch?v=mEeIuTWjdlg
As i stated in my original post, I have swapped out the ECU/PCM/PCU/Whatever with a spare, just to rule that out. I also stated that I performed the mechanical functionality test which is shown in that video, but thanks for posting that up for others to see as well. If anyone else is having similar issues, they may find that helpful.

Quote:
Originally Posted by SemperFiGuy View Post
I don't think the gauge test is going to work in this instance. I could be wrong but doesn't that just test the "mechanicalness" of the gauges by putting them through the motions?
I'm still going with corrosion in the sending unit wire harness somewhere.
In a way, you are correct... That test verifies light functionality, as well as the mechanical ability for the gauges to sweep their full range. But it also does a variable voltage test, where the gauges will sweep in two or three stages (vs. just pegging out and dropping). This will simply verify that the gauges can/are/will read the information being sent to them. The problem in THIS case appears to be a signal issue as most of you are leaning towards as well.

Quote:
Originally Posted by rjbruzan View Post
Does the check gauges light come on?
Ron
Good question Ron, but no.
No CEL, and no codes stored.


Thanks to everyone who has replied at this point....
If anyone is aware of a location in the harness that is prone to corrosion/breakage, let me know.

5+" of lift, 33's, OTK, Armored, Lighted, Chipped, Winched & Locked up Front.
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post #8 of 20 Old 08-15-2016, 12:20 PM
CCKen
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An area prone to wire damage is at the end of the fuel rail. The wire loom gets rubbed through by the end of the rail. Check this area out. See pics.


.

.
If no damage is seen in any of the wire harness runs check the voltage inputs to the OP sensor.

See wiring diagram.

Using a digital Voltmeter...

Remove the OP sensor connector,

Set the voltmeter to 20 VDC,

Turn the ignition switch to ON (engine off),

Touch the body harness connector pin cavity with the Orange wire going to it with the meter + probe and the battery NEG post with the - probe. You should see 5 volts +/- .5 volts.

Touch the body harness connector pin cavity with the Brown/Yellow tracer wire going to it with the meter + probe and the battery NEG post with the - probe. You should see between .015 and .020 volts. If you can't pic this voltage up switch the meter to the 2 VDC scale.

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post #9 of 20 Old 08-15-2016, 12:39 PM
rjbruzan
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When the computer is told low oil pressure from the engine (low pressure or failed sending unit) the check gauges light comes on. As yours does not come on when reading low pressure (as it should with the ignition on and engine off) I would say gauge failure.


Ron

Of all the things Ive lost
The one I miss most is my mind

Its like a fart in a windstorm
It smells good but thats about it

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post #10 of 20 Old 08-15-2016, 03:20 PM Thread Starter
NW99XJ
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Quote:
Originally Posted by CCKen View Post
An area prone to wire damage is at the end of the fuel rail. The wire loom gets rubbed through by the end of the rail. Check this area out. See pics.
If no damage is seen in any of the wire harness runs check the voltage inputs to the OP sensor......
Good stuff right there! ...thanks for that!
I knew the OPS is in the same 5v family of bus sensors that make up the bulk of the OBDII system. But I guess I didnt look thru my FSM closely enough to pick up on the test procedure for sensor voltage.
(kicking myself in the shorts for not thinking of this)


Quote:
Originally Posted by rjbruzan View Post
When the computer is told low oil pressure from the engine (low pressure or failed sending unit) the check gauges light comes on. As yours does not come on when reading low pressure (as it should with the ignition on and engine off) I would say gauge failure.
Ron
The CEL WILL come on (actually its the "check gauges" not "check engine" light that comes on) if I unplug the sensor (gauge reads zero pressure)
But it wont come on if there is even 5psi, like it's reading now.

I still think the last bit of troubleshooting (aside from tracing every inch of wire) is the voltage test and cluster swap.... That will be the definitive test for determining if its the cluster/gauge or not.

Thanks again guys... I'll be sure to report back on my findings.
The voltage test will be tonight, the cluster swap likely this coming weekend.
Keep the advice coming if have an idea that hasn't been covered already.

5+" of lift, 33's, OTK, Armored, Lighted, Chipped, Winched & Locked up Front.
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post #11 of 20 Old 08-15-2016, 11:41 PM
Rob K
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Do you have the right sending unit? There are two kinds, one that operates a light and one that operates a gauge. The sender for a light just turns on or off. I had to change the oil and temp senders when I swapped the light cluster for a gauge cluster. Had to specify that I needed a sender for gauges when at the parts store.

Could also have been a mix up at the parts store and they gave you the wrong one.
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post #12 of 20 Old 08-16-2016, 08:43 AM Thread Starter
NW99XJ
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Quote:
Originally Posted by Rob K View Post
Do you have the right sending unit? There are two kinds, one that operates a light and one that operates a gauge. The sender for a light just turns on or off. I had to change the oil and temp senders when I swapped the light cluster for a gauge cluster. Had to specify that I needed a sender for gauges when at the parts store.
Could also have been a mix up at the parts store and they gave you the wrong one.
Valid concern, but I've got at least two good functioning sending units, (i also have a third I've been keeping as a spare, but i'm not 100% sure of it's condition).... the latest being a genuine Mopar part, direct from the dealer. That's whats in there now.
To my knowledge, the wiring harness is the same throughout the production run
(ie; the harness on a 97 will be virtually identical to a 99)
Just like a sport trim level will still have the wiring for all the stuff a limited would have.
Obviously a 2001 will be a little different just because of the differences in engine management.
But everything else like all the BCM type of stuff will all be the same.
The factory keeps things simple by using the same harness for all trim levels, and as much of it as possible for that model run.
So as far as my experiences have led me to believe, there is no difference in the harness, the pin layout, or the wiring to the cluster.
This is why you’re able to pull an indicator light cluster, and swap it directly for a gauge cluster without having to rewire anything.
Sorry to crap on your point, (and i definitely appreciate the response) but I don’t want anyone else reading this to be confused with misinformation.
With that being said... there IS a difference between the sending units on the older model XJ's and the 97-01 XJ's.
The older model 4.0L engines have a variable resistor to ground for the sending unit, and the later model ones obviously use the 5v+ system like the rest of the OBDII sensors.

5+" of lift, 33's, OTK, Armored, Lighted, Chipped, Winched & Locked up Front.
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post #13 of 20 Old 08-16-2016, 11:57 AM
CCKen
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Actually, the oil pressure sender for the '97/'98 are different from the '99 & up. Even the late '96 is similar to the '97. The '97/'98 OP sender is a two pin device and operates on a different principle than the '99 & up. The '99 & up is a three pin device. I don't think Chrysler prewired the '97 & '98 with the three wire configuration.
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post #14 of 20 Old 08-16-2016, 03:05 PM Thread Starter
NW99XJ
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You sir... are correct.
The 96's were an interesting lot... a bit of a hodge-podge between the upcoming late model technology and the old.
The 99 and up ARE 3 wire, I had forgotten about the two wire version in the earlier ones.

5+" of lift, 33's, OTK, Armored, Lighted, Chipped, Winched & Locked up Front.
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post #15 of 20 Old 08-16-2016, 03:30 PM
Rob K
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For my 1995 all I had to do was install a gauge oil sender in place of the light oil sender when installing the cluster. If I remember right the senders look the same, just different part numbers. The wiring was the same.
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