View Full Version : Jeep 4.0 I-6: Low oil pressure at idle


54FordPanel
08-11-2010, 01:43 PM
I've been working on my daughters 2001 Grand Cherokee this week, and driving it around. I've noticed that the oil pressure, normally 40 lbs at speed, drops all the way down to 10 lbs at idle. I've never seen oil pressure drop quite that low.
Is this a problem that needs addressed? Would be be a weak oil pump, or big bearing clearances due to wear?
It has 98K miles on it.

blaze_125
08-11-2010, 01:58 PM
could be a clogged pickup tube, or even a bad sensor.
I'd start by running 1/3 of a Seafoam can through the crank case for a few miles to see if it can clean things up. Don't let the seafoam in there for too long as it thins the oil.

So yeah... Run a 1/3 of a can for a few miles, then drain the oil.

It worked for me.

bochnak
08-11-2010, 02:02 PM
I would hook it up to another gage just to be sure you are getting a valid reading.

KnightFire
08-11-2010, 02:19 PM
I would hook it up to another gage just to be sure you are getting a valid reading.

x2. Directly plug in a mechanical gauge into the port, Jeep 4.0Ls are notorious for giving false oil psi readings. It could be something as simple as a bad sender, bad pump, or worn bearings. Might as well start with the simplest fixes first :)

What viscosity of oil are you using?

98k miles on a 4.0L is NOTHING :) It should easily go 200k before needing any type of overhaul.

66HertzClone
08-11-2010, 02:21 PM
Take a close look at the sender, they can be very easily damaged when replacing the oil filter.

54FordPanel
08-11-2010, 02:23 PM
I think she runs 10w-30 in it.

But if it was a bad sender, it wouldn't drop down at idle, would it? Wouldn't it be all over the board at any engine speed?

regguy1
08-11-2010, 02:28 PM
I would hook it up to another gage just to be sure you are getting a valid reading.

I agree do this first.

anodyne33
08-11-2010, 02:43 PM
x2. Directly plug in a mechanical gauge into the port, Jeep 4.0Ls are notorious for giving false oil psi readings.


4.0s are also notorious for having low PSI at idle and running for years like they don't even care for oil. As long as you're near 40 by the time you're up to 1500 or 2k RPM it's probably not a big deal. Not ZJ/WJ specific, but there's a TON of 4.0 tech over at www.naxja.org

grcthird
08-11-2010, 02:53 PM
98k miles on a 4.0L is NOTHING :) It should easily go 200k before needing any type of overhaul.

I just put a #4 piston and rod in a 98 Grand Cherokee with 98K on it. The piston skirt was busted past the second ring land and cracked all the way to the face. See pics.

The oil pressure sensor is located right above the oil filter and is easily broke when changing oil, they just break easy. If it's not rattling like hell it's probably ok.

Cummins_Powered
08-11-2010, 02:54 PM
Mine used to do the same thing, and i never did anything about it, and never had a problem.


One thing about the seafoam, is that it is going to loosen a lot of shit, and that shit has to go somewhere...probably into the oil pickup tube.

regguy1
08-11-2010, 03:39 PM
Mine used to do the same thing, and i never did anything about it, and never had a problem.


One thing about the seafoam, is that it is going to loosen a lot of shit, and that shit has to go somewhere...probably into the oil pickup tube.

I would'nt put it in...my opinion

CarCrafter
08-11-2010, 04:07 PM
Hook up the mechanical gauge and see what the story is. I wouldn't be surprised if the engine is tired. The 4.0 will probably go on for another 300K miles like that. I would venture to guess that you don't notice this when the engine is cold but notice it more after you have driven it some. My $.02 is don't use any seafoam either.

wafrederick
08-11-2010, 04:26 PM
Hook up an oil pressure gauge.It could be the gauge itself too.#6 piston is known for breaking most of the time on a 4.0s.

54FordPanel
08-11-2010, 04:37 PM
4.0s are also notorious for having low PSI at idle and running for years like they don't even care for oil. As long as you're near 40 by the time you're up to 1500 or 2k RPM it's probably not a big deal. Not ZJ/WJ specific, but there's a TON of 4.0 tech over at www.naxja.org

There is a ton of info on that site. Jeez, every 10th post seems to be about overheating.......I didn't know that about 4.0 engines. I had one in my '80 Jeep pickup and loved it.

Thanks everybody for the input. I've found 2 posts so far on that site that relate to the same thing. It does sound common, and it may not be a big deal like some of you said.

I guess I need to get an manual, because I'm not sure where the oil pressure sender unit is. There's a large 1" diameter electrical unit in the hole where the distributor would've been.....is that it? There's something above that, but it just looks like a grounding stud.

Time to get a manual.

grcthird
08-11-2010, 04:56 PM
http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Oil-Pressure-Switch-BWD_18971693-P_417_R%7CGRPSESWAMS_545485874___

Mike662
08-11-2010, 05:04 PM
I've had both of the above issues on my wife's 98 Cherokee.

The first time, the wire to the sending unit was loose and the dash gauge read zero all the time (that one was probably my fault since I often disconnect that wire when changing the oil...it's easier to get the filter off).

Fixed that, but the dash gauge has been reading 10-15 psi at idle with a warm engine for probably the last 18 months. Comes up to ~ 40 psi at speed. I've been led to believe that it will probably keep running for a long time like that. Engine has about 150k miles on it.

tdkkart
08-11-2010, 05:19 PM
Factory oil pressure gauges even when working "properly"are notoriously lousy, most are little more than a glorified light.
Problem is, if most people watched a real mechanical gauge, especially one connected with solid metal tubing, it would scare the crap out of them. They'd be convinced that something was about to fall apart in their engine. The oil pressure in alot of engines bounces all over the place, especially at idle, and it varies alot depending on the temp of the engine.

The basis for the 4.0L Jeep engine was designed by AMC for Ramblers way back in the
late 50's. It's only about 1/2 a step above a farm tractor engine. Consequently, there's a bunch of stuff in that engine that are not exactly up to current standards.

ddawg16
08-11-2010, 06:03 PM
If the pressure reading does not jump around...stays steady and climbs up to 30-40 PSI at 2K RPM's or better....sender and gauge are fine....it's just the 4.0.

If you start to hear lifter clatter....then you might have an oil pressure problem....otherwise....don't worry about it....that is just how a 4.0 is.

BTW....on the over heating....the usual cause on the 89 and older Cherokee's was the remote resevoir....I don't think you have that.

grcthird
08-11-2010, 06:44 PM
It's only about 1/2 a step above a farm tractor engine.

:lol_hitti :lol_hitti So true.
I thought the same thing when I dug into that one not long ago. These are super easy engines to work on.

TAftw
08-11-2010, 06:44 PM
Yeah check the sensor. Like 66Hertzclone said, they can be damaged while changing the filter.

Quite honestly if it's running and sounds fine I wouldn't worry about it. My 94 GC just shit the bed with 232k miles on it. That's still pretty low.

54FordPanel
08-11-2010, 07:12 PM
The basis for the 4.0L Jeep engine was designed by AMC for Ramblers way back in the
late 50's. It's only about 1/2 a step above a farm tractor engine. Consequently, there's a bunch of stuff in that engine that are not exactly up to current standards.

You know you could be describing the small block Chevy, one of the best engines ever, and the most successful racing engine ever. :)
It was designed back in the 50's too.

I think the 4.0 liter I-6 is a great engine, my issue notwithstanding. But I know them better as 258 - 6's from my AMC days.

unclethumbtack
08-11-2010, 07:29 PM
Factory oil pressure gauges even when working "properly"are notoriously lousy, most are little more than a glorified light.
Problem is, if most people watched a real mechanical gauge, especially one connected with solid metal tubing, it would scare the crap out of them. They'd be convinced that something was about to fall apart in their engine. The oil pressure in alot of engines bounces all over the place, especially at idle, and it varies alot depending on the temp of the engine.

The basis for the 4.0L Jeep engine was designed by AMC for Ramblers way back in the
late 50's. It's only about 1/2 a step above a farm tractor engine. Consequently, there's a bunch of stuff in that engine that are not exactly up to current standards.

The 4.0 is actually a hot rod 4.2(258). It was originally up until 1991 was a closed loop cooling system and is commonly referred to as a Renix engine (lots of overheating). In 1991 it became an open loop cooling system and was given a little better breathing and was called 4.0 High Output. I absolutely love the 4.0 it is the best engine ever put in a jeep and one of the best and longest lasting engines available. I have 4 4.0's all either in or out of Cherokees (XJ's) right now. Daily driver (130k) offroad rig (150K) Parts rig still running (201k) and the engine I pulled from a wrecked 2001 (19K). I am a big fan of cherokees. One of these days I am going to stroke my off-road rig out to 4.6L. Any time I walk by a cherokee I look in the window to see the mileage, the highest I have seen is 340k.
I digress. I would venture to say that op problem maybe the oil sending unit, mine failed and it showed super high but they can fail either way. There is a couple of really good jeep sites. NAXJA.org and Mallcrawlin.com (more for the grands). If you have any questions shoot me a message.

loudrango
08-11-2010, 09:05 PM
i've had the same problem with my '98 xj with 158k on the clock;goes to 40 psi then i get to a light,etc. it drops to about 10.changed sensor first;same thing.then changed oil pump;same thing so i'm convinced that the gauge is just a crappy internal design.it's runs great with no knocks,so i don't worry about it.

Cummins_Powered
08-11-2010, 11:18 PM
The 4.0 is actually a hot rod 4.2(258). It was originally up until 1991 was a closed loop cooling system and is commonly referred to as a Renix engine (lots of overheating). In 1991 it became an open loop cooling system and was given a little better breathing and was called 4.0 High Output. I absolutely love the 4.0 it is the best engine ever put in a jeep and one of the best and longest lasting engines available. I have 4 4.0's all either in or out of Cherokees (XJ's) right now. Daily driver (130k) offroad rig (150K) Parts rig still running (201k) and the engine I pulled from a wrecked 2001 (19K). I am a big fan of cherokees. One of these days I am going to stroke my off-road rig out to 4.6L. Any time I walk by a cherokee I look in the window to see the mileage, the highest I have seen is 340k.
I digress. I would venture to say that op problem maybe the oil sending unit, mine failed and it showed super high but they can fail either way. There is a couple of really good jeep sites. NAXJA.org and Mallcrawlin.com (more for the grands). If you have any questions shoot me a message.




Mmmmmm I remember my 87 XJ with the closed system! good times, mine actually worked pretty well, especially with a switch for the electric fan:bounce:


I did hate that stupid reservoir though.

nate379
08-11-2010, 11:27 PM
4.0L is ok. Tough, dependable, not that great on power.

I am putting a super clean 80k 4.0 out of my ZJ to swap in a 5.2L. 5.9L would be better, but they are hard to find. (98 only)