Is it possible to just replace the rod and main bearings themselves and that will fix it?

 

rod/mains/cam bearings. as long as the crank/cam is still in spec and smooth enough to throw bearings on them... but while its completely apart why not do it right and do it once? more then likely your crank will need to be resurfaced/polished.

 

You have too low RPM, find out specs and repair RPM problem.
That is why you have low pressure.

 

^^^^^A+^^^^^ set your idle to 650.
"no knocking" I would hate to spend money for a rebuild if I didn't need to. Do not get too obsessed over oil pressure...oil pressure is not everything. If all wear in engine is fairly equal on all bearing surfaces it will have an accumulative effect on pressure even if the wear is within the factory spec limits. For instance if clearances were worn .001 that would still be within spec but would add up to >.017, hence a bigger leak=lower oil pressure. This is where it gets interesting...the lower your oil pressure due to > leakage, the greater the flow. You will get MORE oil to the worn bearings and that is exactly what you want!

 

How do I set my idle higher??

 

have you replaced your CPS? the crank position sensor is normally to blame in the xj for low idle. before i blew the cluth in my 94 sport it was knocking hard and had critically low idle. i thought i killed the engine. but going threw the forums i found that was in fact the problem . i could hardly believe it. once the new cps was in it was purring like a kitten on warm laundry.

 

You did use a gasket on the oil pump didn't you?

 

I got m pressure up a little bit. I jammed a coat hanger in the block through the sending unit hole than ran it for half a second with it removed. It pumped out 3 qts and I changed to 15w50 and a qt of Lucas and its about 5 psi higher. And no I didn't use the gasket for the pump. My friend didn't use his on his 360 and he has decent pressure so I figured I try something different

 

WHY do you want to get your oil pressure up? Think about how the lubrication process works.

 

So oil will get up to the cam and lifters and rockers right?

 

If its got low pressure wouldn't that keep oil from getting to the top end?

 

YES, and YES. You do WANT good oil pressure. (min of 13psi at idle) low oil pressure is from oil bleeding off, and its not flowing at the correct volume/pressure when its low, meaning its starving the mains/rods/lifters/rockers/cam bearings of its needed oil. Now its not a catastrophic failure today but will be with time (I drove mine like this for a year and a half with 15/40 delo) but it will eventually become a big problem(unknown time). My bearings where complete brass when removed (thats a bad thing, 2 layers of aluminum material in the bearings between the crank and the brass in the bearings)

low pressure/high clearance's being a good thing is complete BS. Don't take that advice. if it doesn't have pressure at the sender (before the mains/rods) how could it have sufficient oiling taking place? it doesn't, the oil is bleeding off at the mains/rods and cam bearings, it can bleed off to limit/stop flow to the cam/rods, rockers/lifters ect, all depends on where the oil is bleeding off.

IMO you should have used the gasket, it makes up for the imperfections/roughness in machining, just like a head gasket the metals expand and contract, the gasket makes up for the changes and the slight roughness of the metal.

 

Either I did a poor job of explaining or you have poor comprehension skills. I did not say "low pressure/high clearance's" was a good thing.
What I did say was high clearances will cause low oil pressure....it doesn’t work the other way around. Low pressure is caused by leaks,normal pressure is created by restriction. It is simple physics; less restriction=greater flow. Thick oil will mask the problem by giving a higher gauge reading and give the operator a false sense of security. Your low pressure did not wear out those bearings...your worn out bearings caused the low pressure. If your engine is so worn that oil is not getting to the top end there is nothing you can do other than rebuild it. The height of the oil galley is above the crank,so the mains would be the last to starve for oil, as long as the galley has adequate flow gravity will deliver oil to crank.
The point I am trying to make is the low pressure on your gauge is telling you that it is about time to rebuild....and there is no magic trick to get around it. Go ahead and put thick oil in it and be happy looking at the higher reading on the gauge but you have accomplished absolutely NOTHING.

 

I would agree, I can comprehend, I understand how an engine works

however, if pressure bleeds off whatever is after it will be starving. thick oil will cause more pressure as it bleeds off slower then thin oil, causing the last link to get more oil, not a significant amount but will let the life be a little longer. I think we are on the same page, just looking for more clarification

 

Put the gasket in and see what you have. The pump may well be sucking air.