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Old 04-22-2014, 05:07 PM   #1
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Oil Pressure Dropping

I have a small problem and I have used the search feature here, hopefully properly, and have found similar threads with the same problem but the situation of the jeep isn't quite like mine. Here goes. I have a 2000 4.0L 6cylinder 5-speed manual with about 215,xxx miles on it. The jeep has always had routine maintenance and we have never had any motor problems with it. But, a while back I noticed that the oil pressure kept dropping when I idle. The pressure rises to normal readings when I drive but as soon as I pop it in neutral, or sit at a light, the pressure drops. The oil pump has been replaced, the oil pan has been dropped and cleaned and the oil sending unit has been replaced. Is there anything I can do to fix this? Or is my baby just a ticking time bomb until I have to replace the motor? Any help would be appreciated.

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Old 04-22-2014, 05:19 PM   #2
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Sounds like the main and rod bearings may have too much clearance but I would put a mechanical gauge on it first (even if just temporary) to see if what your electronic gauge is showing is correct or not. If your pressure is indeed low, might be getting close to time for an overhaul. I have a 2000 with 195K on it and still have good oil pressure but the valves are starting to make a little noise (started noticing it about 30K ago). I run 10W30 conventional oil and also add 1 bottle (1/2 qt) of STP and so far have been having good results.

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Old 04-22-2014, 06:18 PM   #3
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Hmmm, interesting. I can't really offer much to help, but I'm subscribing because my 98 does the same thing. I sorta thought it was normal? Looking forward to more responses.
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Old 04-22-2014, 07:23 PM   #4
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Verify oil pressure with a mechanical (wet) gauge. If it is still low at idle then you need new main bearings. The pan needs to come off again. If the main bearing journals are are nice and smooth then just replace the bearings and you will get many, many more miles out of that motor. You can do the rod bearings to while you are at it. It will be about another $65 and an hours work. If the bearing journals are not smooth and flat ot is time for an overhaul. I'll venture to guess you have decent oil pressure at idle when cold. If so this is textbook main bearing wear.

Dove has me thinking... When you say low, do you mean very low as in a low oil pressure warning or it just drops a little lower at idle than when driving? That would be normal.
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Old 04-22-2014, 10:46 PM   #5
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Thanks for the response guys. I'll look into a mechanical gauge this weekend after finals are over with.

For clarification, when I said drops I meant the gauge goes from about 35/40 to 0 or even lower (as if the jeep was off) and then my check gauges light comes on. My apologies on not being more clear.
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Old 04-22-2014, 11:07 PM   #6
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I believe Jerry B and others on the forum have said on a number of occasions that this is normal on the TJ for the oil pressure gauge to go low at idle and increase to normal under engine load. This is exactly what mine does and my motor is in superb condition oil pressure and other wise.
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Old 04-22-2014, 11:07 PM   #7
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Same exact issue here on my 2000 4.0, 74,000 miles, I have already changed out the sending unit with a mopar part. Same issue after change out. I have a mechanical gauge I will be using this weekend to see if it is just a bad gauge.

I would hate to think it is main's with such low mileage, but never now if original owner did good mtce. or not.
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Old 04-22-2014, 11:18 PM   #8
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This is PERFECTLY NORMAL.

Oil, with heat, becomes less viscous. Meaning, It will flow easier and it becomes thinner.

Notice when you first start your jeep that the pressure stays the same... But once it gets hot, the oil pressure will drop at idle. This is because the oil is warm and thinner.

So, why does the pressure fluctuate when you hit the gas? well, your oil pump is on the crank shaft and the faster the crank spins, the faster your oil pump spins and the more oil it will pump into your bearings, raising the pressure.

You'll know your mains are worn when your oil pressure didnt get above 45 EVER. and stayed around 35 when cruising on a hot day and 15 at idle.
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Old 04-22-2014, 11:22 PM   #9
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Originally Posted by TheTank39 View Post
Thanks for the response guys. I'll look into a mechanical gauge this weekend after finals are over with. For clarification, when I said drops I meant the gauge goes from about 35/40 to 0 or even lower (as if the jeep was off) and then my check gauges light comes on. My apologies on not being more clear.
I would look at the sending unit. The pump works or doesn't. A clog getting to the pickup tube screen could do it but never heard of it. I'd stick with sending unit. Going to 0 is not normal. Going lower once warm and high idle is done is very normal our year. About 20 is as low as it gets. Of course RPMs raise the pressure no matter the temp. Was describing as low as you will see. 0 ain't right but if the gauge was your jeep would be dead.
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Old 04-22-2014, 11:54 PM   #10
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Oh, I didn't see your second post. Don't read my above post then.
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Old 04-23-2014, 02:48 PM   #11
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I would look at the sending unit. The pump works or doesn't. A clog getting to the pickup tube screen could do it but never heard of it. I'd stick with sending unit. Going to 0 is not normal. Going lower once warm and high idle is done is very normal our year. About 20 is as low as it gets. Of course RPMs raise the pressure no matter the temp. Was describing as low as you will see. 0 ain't right but if the gauge was your jeep would be dead.
Manjoon, not true on the pump works or it doesn't. All oil pumps can pump at a much higher pressure than normal for the engine at high rpm but don't because of the spring operated oil pressure relief valve. I have rebuilt many engines and usually, if the mains are worn it is because of oil starvation because low oil level or sludge build up, or a bad pump, or just a lot (in this case 215k miles) of driving. This is probably a case of an old motor and the replacing the oil pump was an effort to treat the symptom (low oil pressure) instead of the cause, worn bearings. A worn pump will pump just fine at running rpm vs. idle rpm. If the OP didn't run it dry, the crank should be OK and new bearings will fix the problem. There is no way to know for sure until it is taken apart but I would bet a fair chunk of money on this one.
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Old 04-23-2014, 03:57 PM   #12
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Sending Unit

I had this issue with my 97 TJ with 81K on it. It was the sending unit. Cheap fix. This would be the first thing I would replace.
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Old 04-23-2014, 06:57 PM   #13
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I had this issue with my 97 TJ with 81K on it. It was the sending unit. Cheap fix. This would be the first thing I would replace.
OP replaced the sending unit in his first post.
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Old 04-23-2014, 08:10 PM   #14
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OP replaced the sending unit in his first post.
You have much good advice in your above posts not arguing. Just wouldn't rule out a bad new sending unit. Parts don't work out the box a lot and it confuses the f out of people.
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Old 04-23-2014, 08:18 PM   #15
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My first sensor I bought from the dealership went bad in less than a week, so it is in fact possible that it could be bad right out of the box, even if it is a Mopar part. Not to preclude any of the other information, but I always hook up a mechanical anyway to dispel any possibility of guesswork when it comes to my oil pressure.
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Old 04-23-2014, 08:41 PM   #16
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Thank you all for your help! Like I said, I'll look at using the mechanical gauge once finals are over with, so probably this weekend I'll get to it. I'm really hoping it is just a busted oil pressure gauge in my dash. I don't know how to replace the bearings or where to get them from. Anyone have ideas where I can get them and how much it'll run me?
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Old 04-24-2014, 08:37 AM   #17
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You have much good advice in your above posts not arguing. Just wouldn't rule out a bad new sending unit. Parts don't work out the box a lot and it confuses the f out of people.
So true. I recently replaced the plugs and #2 failed within a month. I did a lot of troubleshooting before replacing that plug. If they were old plugs that would have gone right there and replaced it. New stuff is sometimes defective.
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Old 04-24-2014, 08:38 AM   #18
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Thank you all for your help! Like I said, I'll look at using the mechanical gauge once finals are over with, so probably this weekend I'll get to it. I'm really hoping it is just a busted oil pressure gauge in my dash. I don't know how to replace the bearings or where to get them from. Anyone have ideas where I can get them and how much it'll run me?
$150 for mains, $70 for rods. Throw in another $60 for an Oil pan gasket and oil change. Any auto parts store can get them for you.
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Old 04-24-2014, 11:27 AM   #19
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If you do end up replacing the bearings Pick up some plastiguage & Verify that the clearances are within spec,If the engine has been into before Crankshaft may have been resurfaced & Journals may be smaller than a stock Crank.
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Old 04-24-2014, 12:09 PM   #20
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Do they still stamp oversized bearings? Years ago when I was rebuilding engines, they would stamp the outside of the bearing with the same size (like .015") that the crank journals was ground down.
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Old 04-24-2014, 01:03 PM   #21
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Do they still stamp oversized bearings? Years ago when I was rebuilding engines, they would stamp the outside of the bearing with the same size (like .015") that the crank journals was ground down.
Most brands have a marking (e.g. STD, .010, etc.). If it isn't a number it is a symbol that translates to a number.
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Old 04-24-2014, 04:42 PM   #22
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An amazing fount of knowledge from all of you as usual. My pressure isn't dropping nearly as low as the OP and seems like it's in that normal range the jeep sages stated earlier. But as usual I have learned a lot. Thanks guys, and I'll be watching and hoping the OP gets it worked out.
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Old 05-27-2014, 12:53 PM   #23
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You'll know your mains are worn when your oil pressure didn't get above 45 EVER. and stayed around 35 when cruising on a hot day and 15 at idle.
what will happen when they wear out?
how long do i have ?
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Old 05-27-2014, 02:40 PM   #24
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I added a Autometer electric oil pressure gauge to my 4.0 and with only 70k on the clock. With the oil hot I see about 50 psi at 1900 rpms on the HWY and around 10 psi at idle around 500 rpms.
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Old 05-27-2014, 03:32 PM   #25
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what will happen when they wear out? how long do i have ?
You'll need to get new main bearings...
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Old 05-28-2014, 09:29 AM   #26
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Mine does this and I'm pretty sure it's normal
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Old 05-28-2014, 11:06 AM   #27
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I have a 2000 4.0 sport automatic with 110,000 miles on it and at idle after it warms up oil pressure is around 20 to 25psi. On the highway at 2500 rpm it's 50-60 psi. I think I read someplace that 18-35psi is normal at idle with engine warm.

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