I'm Lost please help!
 

I have a 1990 XJ 4.0 with 250,000 miles. I have been chasseing this oil pressure problem for long enough. I need help.

Here is the problem.
Start up cold 50psi, operating temperature 210 degrees drops to 40psi for about 5min. driving at 2100 rpm drops to 20psi. At Idle drops to 0 psi. No lifter noise at idle. I idled at operating temp for 15 min with 0 psi on the gauge no lifter noise.

The engine sounds and runs great. What would make the gauge read 0?

Here is what I have done.
1) Replaced oil pressure sending unit. No help
2) Replaced new oil pressure sending unit with a mechanical gauge
3) Changed oil with 10W 30
4) Changed oil filter - Mobile One filter.



Any other suggestions?

After you have had it running for a min. look inside valve cover and see if alot of your oil is there. you might have clogged oil ports in the head. does it ever smoke?

Main bearings worn out. Rebuild engine.

I agree your bearings are worn,and or your pump is worn,it would be wise to either rebuild it or find another motor. If it burns up on you it is going to cost alot more to rebuild.

mine acted similar, I replaced the oil pump(napa brand) and all has been fine since.
was easy to do.

It is just worn out. Your oil is getting hotter and thinner so the pressure drops, Change to a heavier oil and your pressure will rise a little. 10W30 is a little thin for summer and especially a worn engine.