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No Oil Pressure on Hot Start Up

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Old 07-22-2010, 04:11 AM
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Default No Oil Pressure on Hot Start Up

Hey guys,

I'm rockin the XJ with the 4.0. On a cold start, the oil PSI is around 60, then once it warms up, it drops down to 30-40ish whilst crusing, then down to 10-15 while idling.

When I turn it off, and come back out relatively quickly (ie: running into a store for 15 minutes) when I first start it up, the oil pressure sits at 0. All I need to do is rev it a bit and it will jump up to what it usually is.

Just curious if any others had this problem?


I didn't have the jeep long enough to notice if this was a problem because the first thing I did when I got it home was an oil/filter change. In the process of attempting to help me with that, my dad broke the oil pressure sender. So I have a new one of those as well.

Next oil change should I try a heavier weight? like 10w-40? Currently has 10w-30...



EDIT: And while I'm talking about little things wrong with the jeep...there's 2 more I'm aware of.

Firstly is the turn signal stalk, it won't cancel after making the turn. Did a search and found there's 2 springs inside the column that are most likely broke. Can I buy those new anywhere?

Secondly, whenever I turn on the air/heat....a loud squeal erupts from the front end of my jeep. Air I could understand because it would be the compressor...but the heat doesn't use the compressor, does it?
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Old 07-22-2010, 07:26 AM
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Gotta say...your post doesnt make sense. You have a low pressure read and, apparently, a broken oil pressure sensor, and are thinking heavier weight oil might help?

Ahh, youth.

I'd go with the loose wires to the sensor diagnoses for the first issue.

Buy a new turn signal stalk at the junkyard for the second...then try to figure out how to install int.

And open the hood...and with a helper to turn the HVAC controls on and off whilst you listen. When you find the squeal, you'll know what is wrong. And yes the compressor can turn on while on heat settings. In some cars, the stupid thing never shuts OFF.
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Old 07-22-2010, 08:42 AM
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first thing to do for the oil pressure is to buy a $8 mechanical gauge and verify what your oil pressure actually is and if it matches what the instrument cluster is reading.
if its low on the mechanical gauge then you have internal engine problems.
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Old 07-22-2010, 09:10 AM
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Originally Posted by Rmart30 View Post
first thing to do for the oil pressure is to buy a $8 mechanical gauge and verify what your oil pressure actually is and if it matches what the instrument cluster is reading.
if its low on the mechanical gauge then you have internal engine problems.
X2 this would be my first step and you can buy the blinker switch from AZ if you cant find one in a JY, the defroster will turn on the AC compressor also...Tj
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Old 07-22-2010, 02:02 PM
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Originally Posted by CAEMI View Post
Gotta say...your post doesnt make sense. You have a low pressure read and, apparently, a broken oil pressure sensor, and are thinking heavier weight oil might help? Hahaha, no. the sender broke and I replaced that already. So I don't think it would be the sender causing the issue since it's brand new.


Buy a new turn signal stalk at the junkyard for the second...then try to figure out how to install int.

And open the hood...and with a helper to turn the HVAC controls on and off whilst you listen. When you find the squeal, you'll know what is wrong. And yes the compressor can turn on while on heat settings. In some cars, the stupid thing never shuts OFF. Huh, I learned something today. Thank you, much good sir.
Originally Posted by Rmart30 View Post
first thing to do for the oil pressure is to buy a $8 mechanical gauge and verify what your oil pressure actually is and if it matches what the instrument cluster is reading.
if its low on the mechanical gauge then you have internal engine problems.
Originally Posted by TrekkerJmm View Post
X2 this would be my first step and you can buy the blinker switch from AZ if you cant find one in a JY, the defroster will turn on the AC compressor also...Tj
Alright, thanks guys. Cheap mech. gauge for oil psi...and turn signal stalk from jy, or blinker switch from AZ. That's weird because searching last night said that there were 2 springs inside the column that sent the blinker to cancel. Whateva! I'm sure the blinker stalk will be easier to replace than pulling the wheel and all that ****, so fine by me.

Oh, another thing on the squealing issue....whenever the squeal starts, my voltage gauge rapidly drops then pegs out down at 7. If I turn the heat/air off it will still stay pegged out. If I turn the vehicle off and then back on, the voltage is totally fine, right where it should be. I thought this was pretty strange as well.
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Old 07-22-2010, 02:47 PM
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Your oil pressure should be lower when your vehicle is fully warmed up. When your engine is cold the oil is much thicker so it takes more pressure to move the oil. When your engine is at operating temperature it is much thinner and easier to move so you will see less pressure on the gauge. That does not mean there is less flow, just that it takes less pressure for the oil to flow. When you hit the gas you will see an increase in oil pressure, but at idle and fully warmed up your oil pressure will be quite low. At idle my Jeep also reads very low oil pressure when fully warmed up.
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Old 07-22-2010, 10:16 PM
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Originally Posted by billulsund View Post
Your oil pressure should be lower when your vehicle is fully warmed up. At idle my Jeep also reads very low oil pressure when fully warmed up.
13 PSI is lowest jeep recommends having at hot idle on the 4.0
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Old 07-22-2010, 11:02 PM
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I agree with everyone else, use a mechanical guage to verify your oil pressure. your turn signal switch (if your jeep is a 97+) doesn't have the springs, that was the older ones (springs are available from GM) the newer jeeps share a signal switch with the first gen Neons....as far as the squeal, if your voltage drops off your belt is slipping, either too loose or something failing.
Good luck!
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Old 07-23-2010, 11:35 AM
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Originally Posted by Rmart30 View Post
13 PSI is lowest jeep recommends having at hot idle on the 4.0
The ideal oil pressure for pretty much any engine is 10 psi per 1000rpm. At lower rpms your engine is not working as hard and needs less oil flow. My Jeep also reads very low at idle (almost nothing on the dash gage) but I also have a low idle (still working on that). I thought I had a problem with my oil pressure so I changed the pressure sending unit, checked and cleaned the pump and pickup tubes and nothing changed. When I had my engine apart I verified that oil was getting everywhere it needed to. Then I read the excellent article below all about motor oil and how it works. It also dispells many common myths. I have come to the conclusion that even though my pressure is quite low at idle there does not seem to be any problems as long as the pressure goes up when my RPMs go up.

http://www.supramania.com/aehaas/index-2.html
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Old 07-24-2010, 07:24 AM
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Originally Posted by billulsund View Post
The ideal oil pressure for pretty much any engine is 10 psi per 1000rpm. At lower rpms your engine is not working as hard and needs less oil flow. My Jeep also reads very low at idle (almost nothing on the dash gage) but I also have a low idle (still working on that). I thought I had a problem with my oil pressure so I changed the pressure sending unit, checked and cleaned the pump and pickup tubes and nothing changed. When I had my engine apart I verified that oil was getting everywhere it needed to. Then I read the excellent article below all about motor oil and how it works. It also dispells many common myths. I have come to the conclusion that even though my pressure is quite low at idle there does not seem to be any problems as long as the pressure goes up when my RPMs go up.

http://www.supramania.com/aehaas/index-2.html

Excellent write-up on oil. Should dispell a lot of misinformation about engine design and oil, especially how the different manufacturers see the problem. I knew some of this stuff anecdotally but this article explains WHY. I read a lot of this type of analysis many years ago published by Shell when their Rotella T oil was first formulated. The one you posted is better. It ought to be a sticky - might end a lot of disagreement.
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Old 07-25-2010, 11:38 AM
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I have the same oil pressure issues, and in fact, at a stoplight, my dash gauge will sometims actually drop to zero, triggering a "check gauges" warning light. A little bit of throttle in neutral, and it pops back up to the first bar on the OP gauge. I was thinking it was an oil pressure sender perhaps, because the drop to zero happens all of a sudden, i.e. I've already been sitting at the stoplight for 30sec with low OP, and all of a sudden it drops to zero, I thought maybe it just happened to drop below a threshold of detection and not actually been zero?? I'll try a mechanical gauge to be sure.

My question is though, what's the likely worst-case scenario? replacing the oil pump?
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Old 07-25-2010, 11:39 AM
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Also, where would a mechanical gauge measure? does it read off the same sender as the dash gauge?
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