High Oil Pressure

I'm new to the forum, joining today. I've owned a 1989 Jeep Cherokee Larado for a couple of years now and really love the car. It's got about 125,000 miles on it now, but still looks in very nice shape and runs fine. Haven't had much in the way of problems with the vehicle, but now I've got a real head scratcher and wonder if anyone on the forum might be able to help me figure it out.

A couple of days ago, I noticed the oil pressure gauge wavering. It normally reads about 40 lb/sq.in. at speed and 20 at idle, but suddenly the pressure started going up to 80, then dropping back to 40. It did this for a while and then parked itself up at the high end of the gauge.

I took it home, changed the oil and filter. No difference. I pulled the sender and replaced it. Still the same. Then I pulled the sender and put a mechanical gauge on the engine (which I probably should have done first!). Oil pressure does read high--a little below what's shown on the dash gauge, but probably between 50 at idle and 70-75 at speed.

Anybody have any suggestions as to where I ought to look next or what might be causing the problem? Is there a downside to driving it while the pressure's up there? Much obliged...

Tom

The pressure relief valve may be sticking inside the oil pump causing the high pressure. I have heard that the 4.0 is prone to oil pump failure around 130,000, never had one fail on me but its an easy replacement, as the oil pan comes off without pulling the engine and only takes about 30 min. As far as the high pressure it shouldnt hurt the engine but might make some seals start leaking.
Hope this is some help, good luck

The pressure relief valve may be sticking inside the oil pump causing the high pressure. I have heard that the 4.0 is prone to oil pump failure around 130,000, never had one fail on me but its an easy replacement, as the oil pan comes off without pulling the engine and only takes about 30 min. As far as the high pressure it shouldnt hurt the engine but might make some seals start leaking.
Hope this is some help, good luck

Rich: Thanks much for your reply. I had a chance to look through the Jeep repair manual the other day, and I'm more puzzled now than before. The manual calls for a normal pressure of between 35 and 75 lbs/sqin at speed and an idle pressure of no less than 16 pounds. Checking the pressure with the mechanical gauge, it definitely doesn't go over 70 pounds, so it appears the pressure relief is working and maximum pressure is normal--assuming you ignore the fact the gauge had been reading 40 pounds at max just last week.

Is there something else that could be causing the high pressure besides the relief valve? I doubt that crud in an oil gallery somewhere is going to cause that kind of pressure. It'd have to be something near the sender or oil filter I'd guess, but I don't know the inside of these engines as well as I know some others.

Tom

I have an 89 cherokee LTD w/4.0 six and normal pressure for it is 65 -70 psi when driving and 45-50 when idleing. It has 135,000 and just started to drop pressure when I rebuilt the engine, reason for pressure drop is unknown but the rod bearings where getting down to the "pink" which is pretty worn out. This truck had been upside down at 32,000 before I bought it, so I figure that maybe the reason. If everything else is running fine I wouldnt worry too much till something else happen.
good luck

Rich: Looking at your note and the oil pressures you list, it's possible then that for some reason my pressure relief valve was obstructed, leading to lower than normal oil pressures, and that somehow, after a couple of years, whatever was preventing it from closing got displaced and the oil pressure is now similar to yours, back to normal in other words. Thanks for your help.

Tom