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Discussion Starter #1
Well my TJ just started doing this about a month ago and I finally went out and bought a brand new oil pressure switch. I put it on yesterday and drove it around today and the CHECK GAUGES light came back on and the oil pressure is at 0. Anything over idle i get oil pressure but never reaches 40.

The gauge problem was usually just occurring every now and then would spring right back up and read that the motor had oil pressure. It started doing it more frequently so I changed out the sensor. Now it does it consistently at hot idle. When I'm driving, the oil pressure is up but never over or close to 40.

I'm not sure what the factory specs are supposed to be on a 97 TJ 4.0 with oil pressure but I put a mechanical gauge on it and upon starting with a cold motor the psi was right at 50psi. Half way warmed up it was at 40. Closer to 200 degrees it was down to 25. After I drove it around the block and really warmed up the oil it was between closers to 10psi on the mechanical gauge.

I then disconnected the mechanical gauge and reinstalled the new oil pressure sensor and reconnected the wiring. I started the jeep back up and the gauge reads 0 at idle. I rev it up and the psi doesn't go over 25. Where with the mechanical gauge it would shot up to 25psi easily.

Any ideas whats going on here? bad oil pump? Change the viscosity of oil (running 10w-40 Castrol)? Bad gauge in the dash cluster?
 

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IMO your oil pump os shot or your engine is worn...?
 

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Discussion Starter #3
But the mechanical gauge is reading far more accurate readings then the stock Jeep gauge.

I'm thinking it maybe a oil pump....I have no knocking, clunking, or any other weird noises internally.
 

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i think you are ok here. especially with NO knocking.

my 4.0 in crawler knocks bad on start-up for about 2-3 seconds then quiets. pressure is 50 PSI cold and then drops to near zero at hot idle, and 10-15 at drive RPM hot. the knock returns at each cold and sometimes hot start-up plus a much softer knock at times while driving, especially at a vertical waterfall climb. i have "milked" this issue for two years now as a trail-only rig while running straight 50 weight oil:shaking::flipoff2:
 

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Discussion Starter #5
rockr I was thinking the same thing. With no internal noises I think it's a faulty TJ gauge.

I pulled the dash yesterday to see if I could locate a loose or bare wire and didn't find anything. I may try 20w-50 oil next oil change and see if that helps. If it doesn't, then I'll pull the pan and check out the pump screen and possibly change out the pump.
 

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Mine dose the same thing also a 97 i have good pressure and no internal problems. i have talked to a lot of people with 97s and most of them do it, it’s a gauge thing i choose to ignore it. seems to start when the vehicle has been offroaded so i think it’s a crack in solder or a loose connection inside the cluster its self but i have been too lazy to pull it apart and trace it to find the problem.
 

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Discussion Starter #7
I guess that's the problem then. I drove it about 20 miles today at 70mph and it held right at 20psi on the stock gauge cluster. Once I pressed in the clutch it drops like a rock to 0 and the CHECK GAUGES light comes on.

I did pull the dash apart though....I couldn't find any bare connections or bad soldering on the cluster.
 

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Hmm

Don't know what factory spec for a 4.0 in a TJ is, but IIRC for a 258, 13psi at a hot idle is considered acceptable.

10 psi indicated might very well be within factory specs.
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Discussion Starter #9
Then I think I'm fine. The mechanical gauge shows me I'm right at 10psi after driving around for 20-30 min. However, the factory dash gauge says I'm at 0 :shaking:
 

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Low oil pressure at start-up or restart only

Frequently on my '01 TJ...upon start-up (or on a re-start when warm), the oil pressure is very slow to rise to normal (a few seconds). Sometimes it rises so slowly that the check engine light comes on and the warning beep goes off a couple times. After a few seconds, the pressure reads normal, the check engine light goes off, and the beeps stop. It doesn't take long to get to normal pressure. It is worrying me that damage is being caused when I fire it up due to lack of lubrication for that short period of time. Or is this just a guage or sending unit issue? Any help would be great. The pressure seems to read normal at all times other than this issue at start-up/re-start. :confused:
 

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Are you having issues with only the oil guage, or all guages? I seem to recall an issue with the tabs on back of stock guage cluster in TJ's. Been too long to remember details. Why not just run a mechanical guage and forget the OEM guage? BTW, from my experience, oil pressure reading will be all over the place on a 4.0L. As long as you have zero knocking, tapping, etc. issues, don't worry about it.
 

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My 97 TJ has been doing the same thing for years now. At first I was worried about it now I just ignore it. Engine runs strong no knocks or noises. I'll be honest with you though when it first happened it scared the shit out of me.
 

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Discussion Starter #13
Just a quick update....

I ended up replacing the oil pump in my Jeep. After I replaced it, the Jeep developed a faint knocking sound coming from within the oil pan. I'm almost positive it's either crank or rod bearings. I do have HIGH, normal oil pressure now though. I have put about 2k miles on this Jeep since and it still starts and runs just fine. I'm sure it's gonna throw a rod someday but I'll wait until it does. I'm swapping in an AW4 in the summer along with a flipped 300.
 

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Are you having issues with only the oil guage, or all guages? I seem to recall an issue with the tabs on back of stock guage cluster in TJ's. Been too long to remember details. Why not just run a mechanical guage and forget the OEM guage? BTW, from my experience, oil pressure reading will be all over the place on a 4.0L. As long as you have zero knocking, tapping, etc. issues, don't worry about it.
On mine, it's just the oil guage. And it's only in that first few seconds upon start-up or re-start. A mechanical guage is a very good idea though. It would surely be more accurate. And as far as knocking and tapping...I have absolutely no knocking. There's a little tapping, which is constant and faint. But this just sounds like normal lifter/valvetrain noise to me. I did some rsearch a while back on the tapping, and everyone seemed to conclude that it was normal with all of the 4.0's.
 

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Again, I have the same issue with my 97 TJ. I replaced the oil-pump, no difference. I have 0 knocking or tapping and havnt been too concerned. Did the mechanical gauge biz too. One problem I do have is I live in Cali and have to smog it soon. I have heard that a "check engine" light is a automatic fail. My Jeep is my DD. I have to come up with a solution soon. I will definately be keeping an eye on this thread.
 
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