How low is too low?

I have a 1992 Limited showing 157xxx miles that is having oil pressure issues. I have read several posts about this and possible remedys, and the 13 psi idle and 4o+ psi at load but have not found anyone posting on how long motor will run with next to zero at idle and pegged gauge at wot. It dont knock, the lifters quieten down in 2 seconds on cold start, but above about 2.5k it does sound like the crank is jumping around a little, more like a rattle than a knock. I have found a few possible replacment motors, but all have over a 100k and I am asking cause I would like to hold out for the right one or maybe build a budget stroker when money and time permit, and enjoy the Jeep in the meantime. I have heard much about low pressure, but most of the engine failures I have seen are from not thrown rods or siezed cranks. Anyone run one for an extended time with low oil pressure?

jeepnfreak said:

....next to zero at idle and pegged gauge at wot......

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Have you tested the oil pressure with a mechanical gauge ? Sounds like you have a oil pressure sender, wiring, loose ground, or gauge issue.

No I havn't tried a mechanical gauge, but I am pretty sure I have problems. I was just out diving it around and it seems that the death rattle is directly proportional to the oil gauge. In other words when the oil gauge starts coming up the rattle (or knock) does too. when taking it easy (below 2k) the engine sounds perfect except the oil gauge is near zero. This was just put back in service and sat for over 2 years and the old gas has and is going through the motor. I thought some of it may have been pinging and I still smell stale gas when fueling up, but it don't sound like pinging. I did run a little seafoam in the oil then changed it. Ran some in the tank too. Did not seem to make any change in sound or pressure. This straight rust free luxury beater with the peeling clear coat eventually will get the motor it deserves I just want to be selective about it. We had snow so the selection has probably just increased. Some folks have said 6 quarts, or thicker oil and dont look back, which sounds good to me, I am just wondering what are the warning signs its about to go. Thanks for the quick reply Don

13psi in the min spec as you listed. If it's not making noise (engine knocking) then as suggested maybe it's the sender.

I'd sure want to verify oil pressure with a mechanical gauge if it were mine. You may very well be right about your engine, but good data = good decisions.

You can rent a mechanical gauge for a few bucks at many parts stores and it will take you about 10 minutes to perform the test.

I have 3 mechanical gauges 2 are installed and I have not been able to find the third, but I will make another effort when it warms up a little. I have been over at youtube listening to some vidieos of 4.0s and mine sounds most simular to the flexplate one, so maybe there is hope for this engine afterall. I have seen so many Jeeps for sale 220k + miles still running great, and delivering the mail is pretty severe duty, mine should just be getting broke in. One other thing I have noticed since first post is upon start up my oil needle is erratic, kinda jumping around. It had a fram, and now it has an STP, I think I will try Oriellys for a WIX, or an AC delco?

I have 3 mechanical gauges 2 are installed and I have not been able to find the third, but I will make another effort when it warms up a little. I have been over at youtube listening to some vidieos of 4.0s and mine sounds most simular to the flexplate one, so maybe there is hope for this engine afterall. I have seen so many Jeeps for sale 220k + miles still running great, and delivering the mail is pretty severe duty, mine should just be getting broke in. One other thing I have noticed since first post is upon start up my oil needle is erratic, kinda jumping around like it pumping air. It had a fram, and now it has an STP, I think I will try Oriellys for a WIX, or an AC delco? Probably be a good idea to change oil filter again anyway after using the Seafoam in the oil.

I have a similiar question. I recently resurrected a '90 Renix 4.0L that had been sitting for close to a year. I have 40psi cold, 20psi warm on the highway, and under 10psi at hot idle. I had an awful rattling noise on cold start (less than 30 degrees) but switching to a horizontal mount oil filter has solved most of that. I put in a new oil pump and rear main (still leaks, just less than before) and the main bearing looked pretty good so I'm just driving and keeping an eye on it.
If you want the most accurate opinion send some engine oil out for an analysis. It will cost somewhere around $30-50 but will tell you exactly what's wearing in the engine.

1987Comanche said:

If you want the most accurate opinion send some engine oil out for an analysis. It will cost somewhere around $30-50 but will tell you exactly what's wearing in the engine.

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If you have metal in the oil and it's not magnetic then it's bearings, if it is magnetic then you've got bigger problems.

I have discovered something new in these warmer days. Mine sounds like flexplate (kind of a random, hollow sounding, stamped metal knock) much like the flexplate videos on youtube. As i repoted earlier my gauge is kinda jumpy, well the new thing I noticed is at the very moment the knock occurs the gauge jumps up. I am wondering if the oil pickup tube may have come loose and gets sucked in at wot and makes the pressure jump up. Any other ideas or suggestions? Has anyone ever heard of, or experienced a pickup tube failure? I bought a replacment motor, but I would rather fix this one and save the other one till I need it. How hard is it to pull the pan on a stock unlifted XJ? This is a 4.0 btw. Thans Don

I only have one experience with this and that is with my 90. I replaced the con rod bearings and it fixed the oil preassure issue. I have over 190K on the original mains.
There is no substitute for good data.

FWIW, I drove with 0 pressure at idle and 15 psi on the highway for 3 months.

Still ran when I took it out and replaced it.

Definitely check oil pressure with a mech gauge, maybe replace the sender if the mech gauge shows good. Also, an oil analysis would help. www.oillab.com

Just to update this. I switched out the oil sender with the donor motor I pulled, pressure went to just over 40psi cold idle, and about 20 after it warms up. No difference in pressure really, but it sure makes me feel better seeing pressure on the gauge. Motivated me enough to pull the inspection plate and check the torque convertor bolts, 3 of them were lose. Well my knock went away too. I'll hang onto the spare engine for just in case, but hopefully it wont be needed for a while. Thx Don