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Old 09-22-2010, 06:25 PM   #1
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low oil pressure at idle

Hi, I am new to this forum and I have a question has anyone else had trouble with the 4.0 losing oil pressure once it is warn and idling it returns as soon as you touch the throttle I have changed the oil and filter twice usung 10w-30 and replaced the oil pressure sensor no better it was fine before I changed the oil other than the oil pressure releif valve I'm stuck

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Old 09-22-2010, 06:31 PM   #2
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What model year TJ do you have and what oil pressure are you getting at idle that is alarming you? Is the oil pressure going up and down in step with engine rpms?

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Old 09-22-2010, 06:58 PM   #3
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I have a 2003 Rubicon I just got it my son has a 1999 Sahara and he was enjoing it so much I had to get one I have been a Mechanic for over 30 years but this is new to me its normal at speed infact at 1000 rpm it comes right up but when it is idling after its warm it goes up slightly them drops right off to the L and the check gauges light comes on
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Old 09-22-2010, 07:00 PM   #4
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Ok, that's a pretty good indication you probably have nothing more wrong than a bad oil pressure sender. Or the electrical plug at the sender is gunked up with oil & dirt & needs to be cleaned out.
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Old 09-22-2010, 07:04 PM   #5
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I have a 2000 wrangler with the 4.0 and my oil pressure is low to me, at hot idle it's about 8 or 9 lbs. And at 2000 rpm driving it is only about 20 lbs. I'm not sure what it should be. I was told it is probably the main bearings. I would also like some insight on this as well..
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Old 09-22-2010, 07:07 PM   #6
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10 psi per 1000 engine rpms is pretty much the acceptable number.
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Old 09-22-2010, 07:17 PM   #7
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Ok, that's a pretty good indication you probably have nothing more wrong than a bad oil pressure sender. Or the electrical plug at the sender is gunked up with oil & dirt & needs to be cleaned out.
Where is the oil pressure sender located so I can clean the plug off, mine is doing this now too.
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Old 09-22-2010, 08:02 PM   #8
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Its on the passenger side behind the oil filter at about the same level you can see it from the top but I reached mine from the bottom it was a bear to get a wrench on
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Old 09-22-2010, 08:07 PM   #9
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by the way the wire and post were clean I have heard that using a NAPA gold oil filter may help I will pick one up tomorrow and try it I have also been told to add STP oil treatmentfirst things first the ruby has 110000 miles on it what bothers me is it was fine until I changed the oil makes me wonder what oil the previous owner had in it. It purrs like a kitten and is smooth and quiet I will put a mechanical oil pressure gauge on it if these cheap fixes don't work got to buy one first
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Old 09-22-2010, 08:43 PM   #10
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Jerry is right. I've been a engine builder for 20 years. I shoot for 15-20 at idle 70-75 wide open and thats on my highend motors that can reach over 30k. I have no problem with a road motor idleing at 8 or 10. in all reality lower pressure is easer on parts(cam gear, dist gear)but need enough to over come bearing load. So if your cruiseing at 2500 and got 20-30lb your fine
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Old 09-22-2010, 08:58 PM   #11
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by the way the wire and post were clean I have heard that using a NAPA gold oil filter may help I will pick one up tomorrow and try it I have also been told to add STP oil treatmentfirst things first the ruby has 110000 miles on it what bothers me is it was fine until I changed the oil makes me wonder what oil the previous owner had in it. It purrs like a kitten and is smooth and quiet I will put a mechanical oil pressure gauge on it if these cheap fixes don't work got to buy one first
from what i've seen fram filters have denser media and will show lower pressure. But it can be a wore motor too. I've seen 20lb difference just by changing a guys filter. I use wix filters religiously. they're sold under other brands too
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Old 09-22-2010, 09:06 PM   #12
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Change the sender before going to the trouble of installing a mechanical oil pressure guage. Changing the sender is similar to changing a spark plug.
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Old 09-23-2010, 05:57 PM   #13
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Ok so today I bought a NAPA gold oil filter (WIX) no help so I removed 1/2 a qt and added STP oil treatment which has seemed to help. Jerry I did install a new Jeep sender yesterday to confirm mine was working so I think the gauge is telling the truth. Once the oil reaches operating temp its like there is a leak in the pickup or a worn pump I can watch it go up and down for a couple of minutes then it will stay down but the engine sound doesn't change. I guess I will drop the oil pan next and replace the pump and pickup thats going to have wait a week or two as money and time are getting tight.
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Old 09-26-2010, 09:43 PM   #14
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Well I put a mechanical oil gauge on her and warm I had 5 lbs of oil pressure at idle when warm here is the kicker the gauge in the car read over half way, so now the oil pan is off and close inspection of the oil pump shows severe wear I have removed a couple of the rod bearings and they look pretty good for 110000 miles there is a minor score line on the crank pin right at the oil hole and on the bearing as well I will polish them with 400 grit and install new bearings more to come as the saga continues
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Old 10-03-2010, 06:53 PM   #15
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OK so now the jeep is back together and a road test today revealed that I have almost no change. 6 new rod bearings a new standard oil pump and when its warm I have 7 lbs of oil pressure and at 55mph I have 19 lbs (this is on a mechanical gauge which I checked against 2 other gauges I have its reading right) I have casteroil 10w-30 and a NAPA gold oil filter anyone got any other ideas. My plan is to drive it and if the engine fails so be it the funny part is that the custer gauge still reads over half I put the old sender back in after the new one began moving up and down and setting off the alarm. Man am I puzzled pressure is restriction to flow all the bearings looked fine the new rod bearings have .002-.003 oil clearance I didn't drop the mains but the rods didn't show any indcation that a main was in trouble. I don't know?????
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Old 10-03-2010, 08:58 PM   #16
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Running too hot?
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Old 10-03-2010, 09:29 PM   #17
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210 when warm
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Old 10-03-2010, 10:07 PM   #18
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My oil pressure is around 40 when at 2000 rpm and around 10-15 when idle is that good?
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Old 01-23-2011, 06:25 AM   #19
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rubicon hobby: did you ever get this resolved? My 2000 TJ with 165,000 miles is having similar problems so I am curious. Mine also runs smooth and quiet...
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Old 01-23-2011, 09:00 AM   #20
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My oil pressure is around 40 when at 2000 rpm and around 10-15 when idle is that good?
That is fine.
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Old 01-24-2011, 05:56 AM   #21
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rubicon hobby: did you ever get this resolved? My 2000 TJ with 165,000 miles is having similar problems so I am curious. Mine also runs smooth and quiet...
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Old 01-24-2011, 06:03 AM   #22
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Sorry Jburke, Not fully awake yet getting ready for work.
The answer is NO I am still driving it i have put 3500 miles on it since I found the problem I have the money for the new engine saved but then I thought lets see how far it will go on this engine I drve it everyday and when you first start it cold it sounds like there is a bunch of marbles in it but it quiets down in 3-5 seconds I took the mechanical gauge out because it was un-nerving me LOL But it is still going Note here I am not stressing the engine only mild offroading for now However I did put a stuck RV out of the sand with her yesterday didn't even break a sweat.
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Old 01-24-2011, 07:16 AM   #23
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That's good!

Mine is quiet when I start it cold. I changed the oil again (10 miles on last oil) but removed Fram filter and replaced with Mobil 1. Also used Castrol high mileage oil and it is better. I havent tried a mechanical gauge yet (just bought it) but the factory one shows 50 when cold, 20 - 30 when at speed, 6-9 at idle. Purrs all the time. I am not going to worry about it just yet either - good plan!

Thanks!
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Old 01-24-2011, 05:36 PM   #24
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I am running Lucas oil stabilizer as well it thickens the oil and boost the oil pressure some. Just remember the engine doesn't need much oil pressure if you don't stress it plus until summer when things heat up you should be fine but sadly she will probably need either new cam bearings or a new engine sooner or later. However before you go down that road buy a mechanical oil pressure gauge and test the system. They are very cheap like 15.00 then you will know for sure where you stand on oil pressure you can even do what I did and put a Tee in the pipe so that you can operate both the dash gauge and the mechanical at the same time for comparision.
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Old 01-24-2011, 08:39 PM   #25
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i have a 99 tj 4.0L. I have had the low oil pressure at idle thing going on forever now. It doesnt seem to effect anything. i dont really know what to do other than pulling the oil pump and adjusting the relif valve. I have disconected the guage in the dash along with the check gauges light and hooked up a standard guage i got at NAPA. The oil pressure is fine cruising down the road but at warm idle it drops to about 10. Here are some simple things u can do to help the problem. Run 15w-40 CASTROL GTX oil, add about 1/2 quart of LUCAS oil treatment, DONT NOT RUN FRAM OIL FILTERS, they tend to not give as good a pressure as a WIXX. you follow these steps and u cant go wrong the pressure will stil drop at idle but not as much its not really that big of a deal. mine has been like this for 30,000 miles and no wear on engine ( i put a bore scope in motor just to check my theory. NOTE: it take 20 LBS of pressure to float the crank and 15 to circulate around the head if you've got that your fine.

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