MJ Comanche Tech. Stock & Modified. MJ (86-92)
All discussion regarding the Jeep Comanche, the pickup truck related to the Cherokee platform.

2.5 to 4.2 engine swap

Old 03-21-2015, 10:35 AM
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Default 2.5 to 4.2 engine swap

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.
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Old 03-21-2015, 01:15 PM
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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.
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Old 03-21-2015, 03:15 PM
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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.
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Old 03-22-2015, 12:38 AM
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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.
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Old 03-22-2015, 08:41 PM
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Originally Posted by andrewmp6 View Post
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.
87 and up, not 88. 86 is the only year the 4.0 wasn't an option.
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Old 03-23-2015, 09:21 AM
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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.
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Old 03-24-2015, 07:18 PM
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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.
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Old 03-25-2015, 11:01 PM
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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.
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Old 03-26-2015, 04:31 PM
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Originally Posted by one_speed_heuer View Post

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.
if it still runs, chances are the rod isnt thrown YET, but is probably making alot of noise
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Old 03-28-2015, 07:21 PM
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Originally Posted by one_speed_heuer View Post
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.
Originally Posted by motorcharge View Post
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.
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.
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Old 03-28-2015, 11:31 PM
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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.
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Old 03-29-2015, 07:33 AM
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Originally Posted by one_speed_heuer View Post
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.
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.
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