During the summer of 2016, I purchased a '93 Jeep Cherokee with a bad motor, the previous owner had busted a piston and cracked the 6th cylinder. My dad and I went to a junkyard and got an engine out of 2001 Cherokee only to find out the infamous 0031 head was cracked but the lower part of the engine was fine. I ended up getting a head off of an '89 Cherokee. Then I got a re-ring overhaul kit (gaskets, bearings, rings) and my dad installed everything. Cranked it up and everything sounded good but after a few minutes, my dad and I noticed that the oil pressure gauge was going down and the engine was getting hot. As the engine got up to around 240-250 F° the oil pressure eventually got down to 10-15 PSI. So I went ahead and bought a new oil pressure sending unit. After installing it there was no change, so my dad hooked up a mechanical gauge and it had the same 10-15 PSI reading. After this, we put another oil pump and pick up tube in it and the pressure went up slightly but it's still sitting around 15-20 and the engine is still running pretty hot.
My XJ has been sitting up ever since I went back to school last August but over Christmas break, we cranked it up and drove it around. The oil pressure seemed to stay around 40 PSI longer in the colder weather but it keeps going down as the engine gets hotter. Now that summer is around the corner I would love to get it running right and take some beach trips with it. My dad thinks there is too much clearance coming from the crankshaft main bearings and wants to go to .002 undersize bearings (it has standard in it right now). He also wants to get another radiator for it. Hindsight is 20/20, we should’ve gotten the crankshaft turned while the engine was apart. I’m open and would appreciate any suggestions/ comments guys!

 

read about plastigage for bearing clearance which should have been done b4...

 

It is typical on these engines to have ~40 psi for oil pressure on start up, and once the engine is completely warmed up it the pressure will drop to under 20 psi. That is normal and due to thermal expansion of the bearing surfaces.

If your engine is actually overheating then that is a cooling system issue and not bearing clearances. You should have a 195* thermostat in there, and it shouldn't run much above 210 unless you are pushing it hard. Check your water pump, thermostat, belt, coolant level and fans. My XJ occassionally ran hot (and always with the A/C on). After replacing the entire cooling system I discovered the electric fan would come on and spin but entirely too slowly. Replaced the fan and solved the problem.

 

I must say, both the xj and zj I own have 150 and almost 200k on the clock, and both hold great oil pressure even when hot. Sagging pressure when hot generally means bad engine wear, I would guess abuse or neglect. Crank isn't the only issue, could be cam bearings, worn oil pump, just not familliar enough with the jeep rebuild to say for sure but generally it means excessive clearances.

 

oil always 40-50 driving, 30 at idle,,170000 miles 2000 4.0

 

same

 

Did you make sure the block wasnt cracked before you swapped heads?

 

plastigauge the bearings and report back. Please don't throw parts at it, test and replace.

http://www.plastigaugeusa.com/how.html

 

The block isn't cracked, we made sure of that. I just ordered some Plastigauge. I'll give y'all an update later in the week. Thanks guys, I appreciate everything