Is it normal on my 98 cherokee that when i start it that the oil pressure is at 40 on the gauge then after warmed up it goes and stays at 10 on the gauge but thats not just it , when i drive it goes to about 35 to 40 while on the gas then when i come to a stop it goes to 10 or even lower on the gauge sometimes to were the check gauge light comes on till i give it some gas? what is the normal pressure suppose to be? it runs fine at all times, if this is a problem where should i start to fix this issue

 

My 98 is kind of the same, I've only had it a few months though. Warm idle is a tad under 20 though and on the gas its slightly above 40. I remember my old 2000 being a bit higher though but it would behave differently as well maybe because it was an auto and my 98 is a 5 speed

 

That's normal for a worn engine or weak oil pump. 10 is below spec for warm idle, Jeep likes to see a minimum of 13.
Oil pressure does start out high when the engine and oil are cold, then drop as the oil and engine warm up, and will increase with RPM.

Are you getting any clatter or noise at idle when the pressure drops?
What weight oil? Brand of filter?
I'd put a known mechanical gauge on and check before getting too worried...the pressure senders do fail.

 

My 2000 does the same thing, however, it was due to worn engine bearings because I didn't catch a blown headgasket in time and coolant mixed with oil is never good. When warm mine will idle @ 10 psi and 35-40 above 1600 rpm. A little on the low side, but it works. What helped me is that I now use only sythetic 10w-30. I have had no issues since the change from 5w and sythetic in the past two years oil pressure wise. My advise is to change the weight and use a good oil and don't forget to keep an eye on the pressure...good luck.

 

5W weight oil is too low viscosity unless you live in very (very) cold climate and use it in winter. I wouldn't recommend anything lower than 10W and higher than 40W. My favorite oil for high mileage engines like mine (175K) is 15-40W Rotella. YMMV.

 

If it's a high mileage engine, you need thicker oil. I'm running full syn 5-40 rotella and it's great for me, but I just recently rolled 100k and I know this motor has been well maintained.

 

We haven't verified the oil pressure with a mechanical gauge, have we?
Could be a bad sending unit.

 

Inaccurate info, this may be your opinion but not a fact. The initial number 5w is not going to affect his low oil pressure at operating temp. He can put 0w or 15w and his operating temp oil pressure wouldn't be affected.

What brand of oil filter are you using and how old is it? Is it the correct size or oversized?

 

In my experience and with those that I have talked with, sending units usually fail either too high or too low, which leads me to believe that your sending unit is fine, but it can't hurt to verify. Use sysnthetic oil in your Jeep, it is well worth the extra $ up front and you get more miles in between oil changes, especially during the hot Summer months. I use Mobil1 10w30 because it works well with the 4.0.

 

To the OP...
My 98 does EXACTLY the same as yours. 10 PSI at hot idle, and around 35-42 at revs. I have no ticking or overheating. However, mine is not some high mileage 4.0. It's a remanufactured ATK longblock with about 8000 miles on it. The low pressure freaked me out at first, but there seem to be no ill effects on mine, even in 100+ degree Texas heat with the AC going full blast. Of course, your experience may vary. I checked my factory service manual, and it says that at hot idle I should have 13 PSI or higher, and 38 -70 at anything over 1600 RPMs. I'm making the assumption right now that my dash gauge may be a bit off and that my idle pressures may be within spec, so at some point I intend to hook up a mechanical gauge just to help me sleep at night.

 

well im at 167k miles on my motor theres no chatter ,clonking nothing it runs strong even with the ac on when i bought it it had a oversize oil filter i dont know why but i put a fram tough gaurd on when i did the oil change i used valvoline high mileage 10w30 im going to do a oil change today and use some of your guys advice as well as check the oil sending unit while im there i noticed oil on the back side corner of the valve cover so im gonna change that gasket too , is 10w30 fine and that same filter good ?

 

also thanks guys im glad to see this is pretty common

 

10/30 is fine. Fram..well... personally for the money I'd buy Wix, Mopar, Purolator.

 

My 98 Cherokee (187,000 miles) was doing the same thing. I removed the oil sender and found oil in the contact chamber on the back of the sender. I jammed a tie - wrap into the hole the sender screws into and ramrodded the passage. I felt a plug of gunk being penetrated as i cleaned the passage out. I replaced the bad sender with a new Mopar replacement from Morris 4 by 4. Now my cold pressure is 55 lbs at idle, 55 lbs at 2500 rpm. Hot pressure on a 99 degree day is 45 lbs at idle and 50 lbs at 2500 rpm.

 

I agree. No orange oil filters for sure. Walmart carries Mopar filters.