Main Bearing replacement

I know im gonna get crucified for even asking this but right now I just dont have the time or the money to do a full rebuild. But can I put new main bearings and rod bearings by just dropping the pan? I need to know cause this is my daily driver and Im suffering from a loss of oil pressure. Once the car is hot the pressure never makes it past 25 lbs on the dash gauge where as it used to sit right at 40 all the time. My RMS leaks and I may have driven it too long before checking the oil level. I just need to be able to drive it for at least 2 months before i can do the rebuild but the oil pressure is a problem. I cant drive it more than 10 miles before the pressure drops. Luckily my job is about a mile from home.

Yes, you can do the bearings.

Note that there are multiple sizes, and they were often mixed when the engines were built new to compensate for manufacturing differences.

You need to get your hands on the FSM for your year, some Plastigauge, and have at it.

Now, the stock oil pumps in these 4.0s are kind of mickey mouse, so a high-volume replacement pump is recommended.

Question: have you checked your oil pressure with a mechanical gauge? The oil pressure sending units are pretty funky, and the dash gauges are a little inaccurate.

Joe Thanks for answering my thread you seem to be the only one that ever answers my threads! I havent check it with a gauge yet but im banking on bad bearings for the fact that I have a pretty substantial knock at start up and its there all the time now. where as before it would quiet down once the engine was warm!

Have you tried tightening the flexplate bolts? They can cause a pretty ugly knocking, though it doesn't really sound like your problem in this case.

Let me know if you really find yourself stuck, I'm only about an hour to an hour and a half away and can usually figure out how to fix something, though I've never done main bearings. During business hours is right out (though I can answer PMs then) but most other times I am free unless you get unlucky and I've made plans.

Try going with a thicker oil. Hope you aren't runinng 5wXX. I find that my rig likes the 10w or 15w the best and it has been that way since day 1.

Kastein and Old_Man have valid points, and another is that the rod and main cap bolts like to work loose, and that will play heck with your oil pressure.

Whenever someone is re-doing their RMS I try to remember to mention to them that they should re-torque the caps.

Joe, I'm curious if you have actually seen that. I haven't. Anything loose enough to lower oil pressure isn't going to run for five more minutes. I've seen you repeat it enough, just wondering what the factual basis is?

Well, I have found numerous engines with loose rod and main cap bolts. No, I have taken no before and after pressure readings to "prove" that loose bearing caps--read that as increased tolerances--resulted in lowering oil pressures. However, as bearings wear, and tolerances increase, oil pressure goes down.

It wasn't my idea to eliminate tab washers from the 4.0 engine, so I guess the Jeep engineers didn't think loose rod and main bearing caps were a problem either. Go figure.

So, next time you need to drop your oil pan, feel free to take some before oil pressure readings and then loosen your rod and main caps a couple turns each and then track your oil pressure for a few months. Be sure and post up the results.

Uh- yeah. Bearings wear, no argument there. Problem is there is going to be absolutely no change in oil pressure whether the bolts/ nuts are at torque or significantly less- unless you take them down to finger tight, at which point everything goes boom and oil pressure is the least of your worries.