Main bearing replacement

[Jeeplander]

I have to replace my rear main seal, I was thinking at the same time replace the main bearings. I have low oil pressure at idle, engine rebuilt 2 yrs ago one 5000km since. I have a mech. oil guage installed. Engine purrs like a kitten, but I don't like the low oil pressure. I replaced the oil pump, oil filter, oil but no change. I'm thinking about trying 15w40 oil next. How hard to change the main bearings?? or any ideas what my trouble could be.??

[Mozman68]

What do you consider low?

[sharpxmen]

What do you consider low?

x2

if you replace the main bearings might as well replace the piston connecting rod ones also

[Jeeplander]

goes below 10psi, this pretty much happened over night. I use 5 to 10% lucas oil treatment and the pressure will stay up above 10 psi. I use napa oil filter. I had a complete master rebuild roughly 2yrs ago.

[neale_rs]

The sender is most likely failing.  Check it with an external gauge before you spend on anything else.

[sharpxmen]

The sender is most likely failing.  Check it with an external gauge before you spend on anything else.

he said he has a mechanical gauge installed

10 psi is low.

[aw12345]

Here is a plan. plastigauge the main bearings if within spec look at cam bearings.
failed rod bearings tend to have little influence on oil pressure since they receive a limited amount of oil. Engines with a spun rod bearing tend to still have acceptable oil pressure. Here is a question though is the pressure relief for the oil pump in the pump it self? Some engines have them located somewhere else and that could be your problem.
On another note, talk to the rebuilder, if they stand behind their products, they should take a look at it, engines should not loose oil pressure 3000 miles after a rebuilt

[sharpxmen]

[...]
failed rod bearings tend to have little influence on oil pressure since they receive a limited amount of oil.
[...]

why do you think they receive a limited amount? - is the same circuit so same pressure, the hole sizes are similar if not the same (goes from the main bearing thru a hole in the crank all the way to the connecting rod bearing), if one loses pressure then the whole system is affected (it is actually the ability to hold pressure rather than the amount of oil).

i'm with you on the pressure valve.

[Jeeplander]

I have replaced the oil sender unit. I replaced the guts of the oil pump,  including the pressure relief valve. Can I check the spacing by removing the bearing caps? I can pretty much do all my own work on the tj, but engine work I don't know much about.

[sharpxmen]

I have replaced the oil sender unit. I replaced the guts of the oil pump,  including the pressure relief valve. Can I check the spacing by removing the bearing caps? I can pretty much do all my own work on the tj, but engine work I don't know much about.

like aw12345 said you can use plastigauge to check the clearance (i think the FSM you can download here has the complete procedure), but that being said you'll still need to determine if it is the bearings or the journals that are the problem (most of the time you can do it visually)

[aw12345]

Well just look at engines with a spun rod bearing the oil pressure loss is minimal, the rod bearings are fed off a hole in the cranckshaft that draws it's oil from the oil film in the main bearing or it gets a shot off oil when the hole in the cranks shaft lines up with the feed hole in the main bearing shell. From many years of experience and hard lessons, the main oil pressure loss is most of the time bad main bearings, a spun or destroyed cam bearing or a problem with the pump or pressure relief valve. Even a cracked oil pump pickup does not give catastrophic oil pressure loss

[sharpxmen]

you can also lose pressure from the cam bearings although that is less likely

Well just look at engines with a spun rod bearing the oil pressure loss is minimal, the rod bearings are fed off a hole in the cranckshaft that draws it's oil from the oil film in the main bearing or it gets a shot off oil when the hole in the cranks shaft lines up with the feed hole in the main bearing shell. From many years of experience and hard lessons, the main oil pressure loss is most of the time bad main bearings, a spun or destroyed cam bearing or a problem with the pump or pressure relief valve. Even a cracked oil pump pickup does not give catastrophic oil pressure loss

some engines do use a splash to the rod bearing as the crank rotates but for the Jeep engine (and most of the modern ones) use constant force feed to the rod bearings, there are annular grooves in the main bearings which constantly feed the oil from the main bearings thru the crank to the connecting rod bearings - any loss on any of the main or rod bearings will result in a bleed of pressure and it is the same circuit will result in drop of pressure in the whole system



this pic and the quote below are from the '95 Jeep YJ FSM

After passing through the filter element, the oil
passes from the center outlet of the filter through an
oil gallery that channels the oil up to the main gallery
which extends the entire length of the block.
Galleries extend downward from the main oil gallery
to the upper shell of each main bearing. The
crankshaft is drilled internally to pass oil from the
main bearing journals (except number 4 main bearing
journal) to the connecting rod journals.

i can't confirm if the holes in the main journals are straight thru or not (i am positive they are but can't tell for sure) - if they weren't then both upper and lower halfs should have a groove in them.