My YJ Has About 55 PSI At First Cold Start Up,But Then It's All Down Hill. After Driving On The Highway For A Bit And Reaching Operating Temperature It Will Eventually Drop To 20 PSI At 6Omph. Once I Get Off The Highway And Come To A Stop The Pressure Goes Down To 0. Even Though It's At 0 The Motor Still Sounds And Runs Fine. I Can't Hear The Lifters Grinding Or Anything. It Ticks Once In A While, But It Always Did. My Gauge Is Fine. I Replaced The Electrical Sender Once And Then Switched To A Mechanical Gauge. Both Read Exactly The Same. Could This Be Caused By A Leaking Timing Cover Gasket And Pulley Seal? Mine Do Leak. Oil Pump Has Less Than 5000 Miles On It. Any Ideas?

NO! It cannot be caused by the leaks you describe! Those leaks are NOT pressure leaks. In order to actually get pressure THAT low you would have to have a MASSIVE pressure leak.
First thing is the lifters..if they do not collapse and sound off you DO have enough oil pressure!!! Somewhere there is a problem other than the pressure..
I am amazed by the mechanical gauge telling you the pressure is low?...
If there IS a failure it would prolly be the pressure release spring in the oil pump.
The description you gave leaves lots of possibilities... But the lifters holding their oil leaves little doubt their IS enough pressure.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

oil pump release spring? i'll look into that. it has me baffled. i know what a motor with 0psi sounds like, and mine is definitely getting oil. i also know a motor without oil pressure wouldn't run for the last few months without a problem. what can i do? could a passage way going to the gauge just be blocked? but that would be worse at start up. runs fine 120k 55 psi at first cold start. drops t 20 eventually at highway speed. then 0 at idle. can i eliminate a crank bearing as being the cause? i'm trying to judge how bad this problem is. thanks a lot for your time jim

If I was sure the mechanical guage was accurate... I might take the receiving guage off of the tubing when the pressure was at ZERO and start her up and let her squirt a quart or so.. Might gwet a lump of something out.. also I would see how much pressure ZERO really was..
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

no such luck. at zero it was spitting out oil, so i mush have some pressure. what is this oil pressure realease spring you mentioned? i'm not familiar with that

First off I would NEVER be afraid of oil pressure unless the lifters collapse! Not in a grocery getter anyway..
The spring in question is the pressure release valve inside the oil pump. It has happened that when the oil is cold and the pressure rises to beyond the spring pressure the valve has opened and STUCK OPEN. Thereby allowing a discharge of oil to KEEP the pressure from blowing out the bearings.
Then when the oil gets hotter and thinner the valve shuts and the pressure doesn't get low from the open valve... If this valve is STUCK OPEN low or VERY low pressure can be observed.
I would first do anything I could think of to assure myself exactly what the pressure is when it reads zero!! If, for sure, the pressure is really at or near zero I would take out the oil pump and look closely at the valve.. If it is open and I can repair it I'd do that.. Yep I am that kind of guy! But in your case I would prolly advise another oil pump.
Back in the day we used to take them valves apart and install stronger or weaker springs.. Most are simply a metal cylinder in a port (hole) with a spring on top held in by a friction pin. The way they work is simple. The pressure from the oil pushes the cylinder (valve) up against the spring.. As the cylinder goes up another hole is uncovered and the oil bypasses OUT of the pump and into the pan. By placing a stronger spring.. (or in some cases streaching the existing spring) the pressure goes UP until it can push the valve to the desired height.
It is not unknown that the springs can break, bind or otherwise fail.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

how many miles?? this sounds like prototypical engine main bearing wear. main bearings are directly related to oil pressure. I generally do not expeirence oil pressure spring failures. Plastiguage your mains and I am pretty sure youll find them bordering on max allowances.