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Unread 12-22-2011, 04:08 PM   #1
qhead72
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4.0 low oil pressure at idle - fixed

Hi everyone, I wanted to pass along my experiences with this. Last week my 2002 WJ 4.0 4x4, 140,000 miles, started having low oil pressure at idle. I was at a stop light and the "check gauges" light came on. I pulled over and checked my oil level and it was good. The oil pressure would rise with rpms but would go back to almost zero at idle. I let it cool off and noticed that pressure was normal on cold start but would slowly drop over a few minutes as the engine warmed up. I searched here and on the web and found lots of info. I first tried a replacement sender but there was no change. I cleaned the connector with electrical cleaner since there was some oil around the pins - no change. I dug around and found an old mechanical oil pressure gauge and installed it and left it hanging in the engine compartment. Showed same as the dash gauge - low pressure, about 8-10 psi at idle and rising to 55-60 at 2500 rpm. I let it idle for a while but heard no odd noises. Just the same as before - good pressure on cold start and decreasing as engine warms. Went back and searched some more and found info about the oil gallery behind the sending unit getting clogged with sludge. So I took off the mechanical gauge and left the brass adapter in the hole and stuck a short piece of rubber hose to that and into a drain pan. I then turned it on and revved it at 4000 for about 8 seconds and shut it off. I checked the drain pan and found some obvious bits of sludge that had been blown out. I connected the mech gauge again and topped off the oil. Turned it back on and problem solved! Pressure at idle now 18-20 psi and 60 psi at 2500. Replaced original sender and dash gauge was back to normal. Hope this helps someone with the same problem.

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Unread 12-23-2011, 05:16 PM   #2
cruiser54
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Great to hear it!! I wonder if you couldn't use a pipe cleaner down that hole and get a similar result?
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Unread 12-24-2011, 05:02 AM   #3
tjwalker
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ghead72,

That is VERY interesting; don't know if I have ever heard of that one before.

Thanks for passing this useful information along!

Gotta ask to try to understand the "sludge" thing. What is the history of oil changes on this engine?

IE: did it get regular oil changes or not so much? If there was sludge in this gallery, there may be sludge in other parts of the engine. It might be interesting to pop the valve cover off and see if there is a sludge buildup on the valve train.
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Unread 12-25-2011, 05:17 AM   #4
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This really got me curious. I went out to the garage and looked at my stroker block on the engine stand. Nearest as I can tell, the blockage must have been in the brass adapter as the threads into the block for the adapter are at least 1/4" pipe if not 3/8" pipe. Anyway, the tip of my pinky fit in there easily.

The bottleneck for oil to sludge up must be the adapter. Could be a worthwhile suggestion to those reading low oil pressure with no noises to remove the adapter and clean it out?

If a sludgy crankcase is suspected, a guy can shine a flashlight into the oil filler hole in the valve cover and see the amount of sludge in the top of the rocker arms. I've been doing this periodically on my wife's motor as I've been adding Marvel Mystery Oil to the Rotella to clean it up. BTW, it's working very well.
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Unread 12-25-2011, 10:43 AM   #5
dukie564
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hmm that's interesting...

makes sense though - if an engine is babied though it's whole life chances are there will be buildups and i'll get lazy. I purposely get mine over 4000rpm at least once or twice a week to keep it strong. At 250k miles it has tons of power and runs great.
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Unread 12-25-2011, 11:22 AM   #6
cruiser54
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I'm not sure it's an RPM related failure. I think it may have more to do with infrequent oil changes and the fact that any sludgy oil would have the ability to go up a dead-end passage and cool in a brass fitting.
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Unread 01-16-2012, 05:06 PM   #7
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i was told the sending unit was good. i'll try to blow that all out. so, i just unplug the sending unit and turn on the vehicle and let it all get pushed out into a bucket or something? i've changed the oil fairly regularly. might have gone over a couple times, but it's always had the proper amount.
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Unread 01-17-2012, 04:28 AM   #8
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I don't know what's the best way. You could remove the 90* fitting and clean it. That would give you a chance to look into the larger hole going into the block.
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Unread 01-18-2012, 05:25 PM   #9
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I just read on another froum that a guy fixed two XJs with this problem by removing the oil sender and running a pipe cleaner through the elbow. That's easy.
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Unread 01-19-2012, 02:59 PM   #10
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i just added BG109 to my crankcase, ran in for 15 minutes, drained the oil, replaced with a new oil filter and oil, and now, when my jeep is up to running temperature ,210degrees, and in idle or park, the oil pressure sits between 11 and 14 degrees, i would say. before, at that temperature, it dropped from below 10 to almost zero. i would like to think that i just needed an engine flush and a new oil filter. it seems to have done the job. surely, i don't need a new engine already, like a mechanic told me, with less than 100k miles on it. the last mechanic i talked to told me to do what i just did and it seems to be right now. does anyone think that this is something that might come back to haunt me, or is it fixed?
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Unread 01-19-2012, 04:23 PM   #11
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Quote:
Originally Posted by bryanjoyce View Post
i just added BG109 to my crankcase, ran in for 15 minutes, drained the oil, replaced with a new oil filter and oil, and now, when my jeep is up to running temperature ,210degrees, and in idle or park, the oil pressure sits between 11 and 14 degrees, i would say. before, at that temperature, it dropped from below 10 to almost zero. i would like to think that i just needed an engine flush and a new oil filter. it seems to have done the job. surely, i don't need a new engine already, like a mechanic told me, with less than 100k miles on it. the last mechanic i talked to told me to do what i just did and it seems to be right now. does anyone think that this is something that might come back to haunt me, or is it fixed?
Might be worthwhile to do the pipe cleaner thing then.
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Unread 01-21-2012, 06:36 AM   #12
WIRED4U
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low oil pressure at idle

I appreciate your informative post. I pulled the sending unit and elbow, ran a zip tie thru the exposed passages and replaced the jiffy lube filter with a mobil 1, cleaned the oily residue out of the inside of the sensor connector and reassembled. This fixed my identical problem.
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Unread 01-21-2012, 06:48 AM   #13
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Quote:
Originally Posted by WIRED4U View Post
I appreciate your informative post. I pulled the sending unit and elbow, ran a zip tie thru the exposed passages and replaced the jiffy lube filter with a mobil 1, cleaned the oily residue out of the inside of the sensor connector and reassembled. This fixed my identical problem.
That's good to hear. I commend the OP for discovering this "cure" and making the effort to get the word out.
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Unread 01-21-2012, 07:31 AM   #14
99WJCHRIS
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Im having this same problem right now. Thought maybe the sending unit was bad. Ill try this out first.

I've got some seafoam i was thinking about running through since im due for an oil change anyway. What's yalls opinions on that? 99WJ 4.0 W/ 195,000
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Unread 01-21-2012, 12:29 PM   #15
cruiser54
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Quote:
Originally Posted by 99WJCHRIS View Post
Im having this same problem right now. Thought maybe the sending unit was bad. Ill try this out first.

I've got some seafoam i was thinking about running through since im due for an oil change anyway. What's yalls opinions on that? 99WJ 4.0 W/ 195,000
I'd take a gander through the oil filler hole and see what it looks like in there. I prefer MMO in the crankcase for awhile over Seafoam.
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