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Anyone know how to change main bearings?

WebDog

WebDog

NAXJA Forum User
Location
Center City, MN
I just got my 90 XJ running again, apparently the PO removed the thermostat and that was causing it to not heat up.

So I installed one and now it runs about 210 - 220 but the oil pressure is almost non existent, 5 psi at idle and 10 - 20 psi under load.

I have a rattle from the engine and the smog guy who failed me said that it was the main bearings, but I searched for it and cant seem to find how to replace the bearings.
 
If you want it done right, take it to a good shop. Main bearings is not an average joe kind of job, to do it right requires time and accurate measuring. If you need to save money and want to risk it, just buy a set and slap them in. They are inside the oil pan...drop pan, drop crank, replace bearings, reassemble.
 
x2 on a shop doing it, by checking all the tolerances.

the tolerances have to be checked to match the bearings up to each main saddle and bearing cap and crank journal . Otherwise the overhaul could yield low oil pressure, or spun bearings. They also need to crush very slightly when the main cap is torqued down.

If you tackle it yourself, you can pull a main cap, drop the lower bearing shell, and drive the upper bearing shell out of the main saddle using a brass rod, much like changing a rear main seal. But it's a shot in the dark, it could end up worse than before if the bearing clearance isn't matched up.
 
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First things first, Is to add a few Very key ground straps/cables to the rig.

I put a brand new long block in my rig a few yrs back (1990) and the oil showed very low like that. I just lived with it knowing that it was just wrong. then about 6 months later I did all new power and ground cables and added many new ones (grounds) it jumpes up the oil pressure reading to right where it should be after that. Unfortunately the gauge is elect and not mechanical. So the voltage it see's is very important.
 
ROBERTK said:
First things first, Is to add a few Very key ground straps/cables to the rig.

I put a brand new long block in my rig a few yrs back (1990) and the oil showed very low like that. I just lived with it knowing that it was just wrong. then about 6 months later I did all new power and ground cables and added many new ones (grounds) it jumpes up the oil pressure reading to right where it should be after that. Unfortunately the gauge is elect and not mechanical. So the voltage it see's is very important.
Click to expand...

Yea i do plan on testing the pressure with a mechanical one...

The pressure used to sit around 30 - 40 psi but the engine never got up to operating temp.

Should i try a thicker oil?
 
WebDog said:
Yea i do plan on testing the pressure with a mechanical one...

The pressure used to sit around 30 - 40 psi but the engine never got up to operating temp.

Should i try a thicker oil?
Click to expand...
A thicker oil is only a temporary bandaid at best. If the clearances are too loose, it will wear out anyway. It may be cost effective to get a short block because then you are getting a timing chain and cam bearings etc. If you are on a tight budget (who isn't) you could just replace the main bearings but you should expect other things to wear out as well. It may end up cheaper in the long run to just install a fresh short block. Obviously you need to have the head evaluated by a machine shop as well to see if it needs to be rebuilt.
 
Well I am just trying to get it running and registered, trying to pass smog.

The tapping is present in my other XJ as well, but not as loud. So i am not sure wether to just say its a jeep noise since they make a lot or if it is something i need to really worry about.
 
Definitely check true oil pressure first, then go from there. Could be sending unit or something in between. For what it might cost to do the bearings, you might be able to pick up a long block or complete engine for the price and swap that.
 
Im really hoping it does not come to a new engine, I got this XJ to be a second DD and already gone way over budget. But the suspension and everything else i did right this time and not halfassed like my other XJ.
 
The tapping noise is basically an XJ thing. I would do a oil pressure check, if that's good then do a compression check, if that's good then you are probably O.K. and your smog issue is probably elsewhere.
 
jeeperjohn said:
The tapping noise is basically an XJ thing. I would do a oil pressure check, if that's good then do a compression check, if that's good then you are probably O.K. and your smog issue is probably elsewhere.
Click to expand...

I think it will pass smog now, It failed because of to much fuel running rich.

The PO removed the thermostat and the engine never got over 120 - 140 degrees and so the coolant temp sensor most likely was always in the warmup stage causing it to richen the mixture up.

With a thermostat installed the engine runs 210 - 220 like my other XJ.
 
FWIW, I pulled the engine out of my 94 some 6 years ago, over 200k on the engine. While it was out, I put in a new set of bearings, a high volume oil pump, front seal, and rear main seal. The block didn't show any oversize or undersize stamps, so I went with a standard size set, and PlastiGage'd every one on installation. It was my DD for the year following, and no issues have come up since.

It's really not that difficult to do if you have the engine out, and assuming that you don't have any over/under sizing problems. You do have to be detail oriented, and a helper to pull and reinstall the crankshaft would be a good idea.
 
if its a KNOCKing noise, check your Flexplate / flywheel bolts and look for any cracks.
 
If you search the oil pressure threads, someone else got his oil pressure up by running 50W. It will help for a while. And your in CA so you shouldn't have the cold start up problem. If you have low oil pressure, ALL your bearings are probably worn out. CAM, MAIN and CON ROD. Just doing the mains probably is a lot of work for nothing. If you can't do it yourself, try to find a low mileage junkyard motor. Or one in a wreck that still runs.
 
Well i put a quart of lucas and then 5 quarts of 20w50 and it seems to be holding 20 psi during idle and then 40 psi under driving load.

My plan now is to drive it until it blows up as i don't have money for a rebuilt engine right now.

It also passed smog today, so that is good news.
 
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