one_speed_heuer

Right now Im gathering the money to buy a 88 MJ with a 2.5 5 speed 4x4 with a thrown rod. My dad has a rusted out 88 or 89 YJ with a 4.2 and 5 speed. Does anyone know if I could swap the engine and transmission out of the YJ into the MJ or would it be better to find a 2.5 and replace the engine?

Ive heard to swap a 4.0 into some Comanches you have to modify the firewall, and if I remember right the 4.2 is very similar to a 4.0. Yes I know the 5 speed out of the YJ is the "crappy" one. Its free until I can find an ax 15.

Crazy 8s

The 4.0 should fit just fine in your MJ but you will be making quite a few changes. The firewall should fit the 4.0 just fine...and most likely the 4.2 instead. I don't know enough about the 4.2 to know how it is different from the 4.0, aside from displacement.

Metmknice

Sell/junk the 4.2L And swap in a SBC or 4.0L. Easy and better than 4.2L

88 already has the larger engine bay/modified firewall. So, other than some wiring, the 4.0L should be plug n pray.

andrewmp6

Yeah the 87 and back xj/mj had the smaller engine bay,The 88 and newer had the larger engine bay and came with the 4.0 stock as a option.Like said go 4.0 or v8 anything else wouldn't be worth the time of the swap.

motorcharge

87 and up, not 88. 86 is the only year the 4.0 wasn't an option.

one_speed_heuer

Is the 4.2 really that bad? I remember when the YJ was running before the frame really went bad, there wasn't much it wouldn't do. It's carbed so I figure it doesn't have all the electronics to worry about making it a fairly easy swap. If it'll fit.

Main reason I ask is that the 4.2 is free. Another engine and transmission isn't. I know it won't be my 4.0 ho.

motorcharge

4.2 is a solid motor but most people here aren't familiar with it. I wouldn't hesitate to swap one in over a 4 banger if I had one laying around.

one_speed_heuer

Ive been looking at upgrading a 4.2. Ive decided if the 2.5 is completely shot. Im going for the 4.2(its supposed to have optimum torque around 2500 rpm instead of the 4.0 at 4000 or so.) Plus its free, and unique.

According to the guy with the MJ. The 2.5 does still run, I thought a thrown rod normally disabled the engine but I guess I can figure it out, either way Ill be happy.

dnuccio

if it still runs, chances are the rod isnt thrown YET, but is probably making alot of noise

Outlaw Star

The 4.2 is a great motor. Most here are too young to remember or have owned the original 258 (4.2L) the 4.0 is a destroyed, EFI version of the 258. 242cid from 258cid. If you have it, use it.

one_speed_heuer

If the 2.5 is repairable with basic skills I have, I'll get it fixed. Otherwise it's been like 3 years since the 4.2 ran. But it shouldn't be too much of an issue.

Outlaw Star

I'm rebuilding my 2.5L with a 2.9L stroker kit and cam. It is fairly easy to work on and the price of all components was no more than a stock rebuild kit.