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Oil Pressure on my 04 4.0 drops to nothing after reaching operation temps

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Oil Pressure on my 04 4.0 drops to nothing after reaching operation temps

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Old 11-03-2018, 06:44 PM
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Default Oil Pressure on my 04 4.0 drops to nothing after reaching operation temps

I have a 2004 jgc Laredo 4.0 and the damn thing works great unless I have to drive it for more than 30 minutes, if I drive continuously for more than 30 minutes to an hour, then whenever I come to a stop and my rpms go down, I loose most if not all oil pressure. until I take off again or rev the motor in which case itís just fine. Why would that do this. Donít oil pumps go out in a bang and not gradually? Please inform me of wtf I gotta do.
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Old 11-04-2018, 06:42 PM
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Welcome to CF! Please change your profile to read 2004 Grand Cherokee (WJ) 4.0L. It will help with future posts.

It may be your oil pressure sensor. I say "sensor" because it's not a simple pressure switch, but a complicated digital chip-driven sensor. Pull out your wallet and buy a Mopar sensor, don't waste your money on an aftermarket piece of crap. I've already been thru this with my 2000 WJ. If you're not getting bearing rattle, you've got oil pressure.

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Old 11-05-2018, 07:33 AM
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Its either you really don't have oil pressure, or you do and just can't see it. My guess is you don't. However on the long shot, much has been written about clogs of old oil getting in the passage way to the sensor, as well as bad sensors.
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Old 11-05-2018, 07:40 PM
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BTW, IDK if this is the place to say this, but I just switched from Valvoline dino 10W-30 to Rotella T6 5W-40 after 245K miles on the engine and I gained 5 psi pressure in all aspects of operation. That is, there's 5 lbs more at any temp or speed than there was before. Just though you'd like to know.
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Old 11-06-2018, 02:50 AM
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Originally Posted by dave1123 View Post
BTW, IDK if this is the place to say this, but I just switched from Valvoline dino 10W-30 to Rotella T6 5W-40 after 245K miles on the engine and I gained 5 psi pressure in all aspects of operation. That is, there's 5 lbs more at any temp or speed than there was before. Just though you'd like to know.
Yeah this is the place to say that. Really? Even when hot hot hot?
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Old 11-06-2018, 05:45 AM
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At 210*, it's still holding 40 psi at idle. Almost 60 at 70 mph!
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Old 11-06-2018, 06:57 AM
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Hm. This is the WJ?
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Old 11-06-2018, 10:58 AM
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Yeah, the one with 245K miles on it and a supposedly cracked piston skirt. Amazing ain't it? I switched to Rotella because it's supposed to be ashless when it burns so I thought I'd see if it fouls that plug less. My CCV oil separator is working perfectly. It pulls out about 1/2 cup of oil every couple of weeks. That's what was going thru the intake and carboning up the intake valves.
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Old 11-06-2018, 11:01 AM
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You had a valve job on that and it still pulls oil into the catch can?
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Old 11-06-2018, 11:44 AM
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You can always hook up a mechanical oil pressure gauge temproarily to that port and see what's really going on. The Oil Pressure sensors on the 4.0L engines are a known weak spot they go out of calibration, leak, or just plain do wacky stuff and something as simple as hitting it during an oil filter change can damage it.

Aftermarket sensors are a waste of money. I've gotten them bad straight out of the box. I had one that would peg the gauge at RPMs over 3000 for some reason but would read normalish oil pressure at anything less. Dropped in a Mopar sensor that was about twice the cost of the SMP sensor and no more issues.
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Old 11-06-2018, 12:09 PM
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Originally Posted by dave1123 View Post
BTW, IDK if this is the place to say this, but I just switched from Valvoline dino 10W-30 to Rotella T6 5W-40 after 245K miles on the engine and I gained 5 psi pressure in all aspects of operation. That is, there's 5 lbs more at any temp or speed than there was before. Just though you'd like to know.
I had the same results when I went from 10w30 to 15w40 diesel oil, 5-10 psi greater oil pressure across the entire rpm range.
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Old 11-08-2018, 07:07 AM
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Well, with 15W-40, I would expect it!
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