low or no oil pressure

91 4.0HO ax15 223k
so a while ago my oil pressure started reading much higher than previously. it was at 60-80psi on the highway and maybe 40psi idle. before that, it was an average 20psi idle and 40psi highway.
now, all of a sudden it reads 20-30psi highway and zero at low rpm's.
motor runs fine, no noises. the rear main seal and oil/filter was changed about a week ago and all bearings, cam, and pan looked good and relatively clean. the oil pickup screen was clear. no sludge anywhere, just some varnish. i put in 10w30 synthetic, same as last time. 6 quarts.
my guess is sensor, gauge, or wiring issues. anyone else ever encounter this? i want to throw a mechanical gauge on it before i drive it again i think. or at least check for loose/corroded wires at the sensor.

Sounds like it might be a bad sending unit (sensor).

my sending did that same thing. i put a new one on and it reads fine now.

went out to start it and let it get up to temp and it reads good. maybe wiggling the connector did the trick. also found my temp sensor wire was disconnected. now i have a temp gage!

When you have concerns about any engine's oil pressure RUN, don't walk or drive, to get your hands on a mechanical oil pressure gauge and then CONFIRM the actual oil pressure.

Fram filter?

old_man said:

Fram filter?

Click to expand...

Tee-hee. Could be.

Ok now my oil pressure is very low. I have changed the sending unit, no dice. I changed the rear main and the entire pan was very clean, as was the oil pickup. Main bearings looked to be in great shape, as did the cam lobes. It is zero psi at idle, maybe 20psi driving around. I do believe the pressure is actually low because i hear the lifters at times, and the exhaust note changes due to the valves not opening completely. I tried thicker oil because i had been running 10w30 synthetic, but no change. I have tried changing the filter, no change. Same with the sending unit. It does not consume much or any oil. No Fram filters, ever. Any ideas before i just run this thing into the ground? The lifter noise seems to come around more when the engine hasnt been running for very long, especially after hard cornering or hard braking (i flog the crap out of this thing, it's always revving 3-5k). I have 5.38's so it constantly sits at 3100-3500rpm's on the highway. It's a pig so it doesnt move under 3k, so i shift really late.
Oil is full and clean/new. No loss of coolant or milky oil. I did get to 260f once recently when my radiator blew on the highway, but i dont think it ran it very long. Otherwise, runs 200-210f.
91 4.0 ax15

joe_peters said:

When you have concerns about any engine's oil pressure RUN, don't walk or drive, to get your hands on a mechanical oil pressure gauge and then CONFIRM the actual oil pressure.

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This. If you verify with a mechanical gauge that the oil pressure is bad...

If you aren't finding metal debris or seeing any bearing wear, I'd look at the oil pump. A little wear or defect in the pump won't put out so much debris as bearings but will still drop your pressure.

Before you really get into this, though, you NEED to check oil pressure with a mechanical gauge. Sending units go out, bad parts get sold in new boxes, etc. Don't make major decisions about your motor based on what you see on the dash, old sending unit or new.

Not to be one of those guys who asks advice then argues, but the oil pressure IS low, don't need a gauge for that. it sounds like a cummins now, absolutely no oil pressure. i'm going to drop the pan and check it out again. Any other ideas besides the pump? is that common with a higher mileage 4.0?

Since you are pulling the pan, might be a good time to swap out the oil pump. See if it makes a difference. Just because it was "clean" doesn't mean its operating properly. Thought about pulling the VC and let it run see if oil is reaching up top? If there really is little/no oil pressure meow, bearings could be toast..time for a rebuild or V8 swap!

Vanimal said:

Not to be one of those guys who asks advice then argues, but the oil pressure IS low, don't need a gauge for that. it sounds like a cummins now, absolutely no oil pressure. i'm going to drop the pan and check it out again. Any other ideas besides the pump? is that common with a higher mileage 4.0?

Click to expand...

I had to install a high flow pump at 235,000 miles and switch to 20W50 oil on mine. I am at 282,000 right now.

Fram??????

definitely no Fram, and i tried changing the filter a couple times. no change whatsoever. i am running 20w50 now, no change either.

problem solved. oil pump was cracked. wasnt sealing against the block properly.

new pump and rod bearings. 50-65psi all the time now. none of the bearings spun but all rod bearings were into the copper. crank was ok. took a while for the lifter noise to go away, but it did and now it's mostly quiet. still more noise than before this problem, but it's still quieter than most 4.0's i've heard.
fortunately these things are stupid easy to work on...

Thanks for the up date. Since I don't use Fram, and have a new sender, at least I know the symptoms of an oil pump problem.

I know I could search, but how hard is the pan removal on an XJ?

Thanks again Vanimal:clap:

Mine was probably a freak accident. I wouldnt count on yours being cracked.
Oil pan on mine is super easy since i have about 5" of lift. I don't even have to jack up the jeep or disconnect anything for the entire job. the biggest part of the whole job is unbolting and bolting in the oil pan since there are so many bolts.

Glad you got the problem solved!