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Discussion Starter · #1 ·
I have a 89 yj with a 4.2, and have a 4.0 injected for it. Getting mixed info, some people say don't swap, just fuel inject the 4.2, and some say swap them?? What should i do?
 

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If its an HO 4.0, I would take the head, intake, & exhaust manifolds and put them on the 4.2. I think HO's were 1992 and newer or there is info of what serial number to look for on the head.
 

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The question is, what do you want to do?

the 4.2L is a 258 CID, and the 4.0L is a 242 CID, so not much change there. You will lose a little with the loss of 16 cubic inches.

the 4.2 is a low end torque motor where as the 4.0 is more of a mid range motor.

If you are looking to fuel inject the 4.2, its around 1500 complete, I have a 4.2L in my 87 and i looked into this option when i replaced the motor with a crate motor. BUT, i am old school, so i went with an Offenhauser intake and a 4bbl carb. Plus numerous other upgrades that i wont get into.

The fuel injected motor is nice when everything is working ok, but they suck when they arent. It has to have a computer in order to run, and therefore requires wiring and other parts that you dont currently have in your harness.

So, its really up to what you want. And how much time and money you want to spend
 

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Discussion Starter · #4 ·
Thank you, I'm interested in just swapping the head, intake and exhaust. But don't know much about all that, will it just swap over? Will any of it need to be machined? And what about the wiring harness for the fuel injection?
 

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The head should definately be sent to a machine shop and checked for warpage as well as magnafluxed to look for hairline cracks.

You will need the computer that went with the 4.0 motor in order to use the intake and fuel injection. It wont work without it. Same with the engine bay harness, it is possible to splice into or add a fuel rail harness, but much easier if you have the harness from a donor vehicle.
 

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Discussion Starter · #6 ·
Thank you, the nice thing is i do have both harnesses from the donor cherokee, but just swapping that stuff will mean not all the harnesses will be used? Or should I say, not all components of the harnesses. Sounds kinda technical
 

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Also, if you put 4.0 head on 4.2 there is water jackets or ports that need to be filled on 4.0 head. There are many write ups on what to do. Also, you may know this already, but I hear either intake manifolds will fit both heads. If you go with 4.0 head, you must use exhaust manifold or header from a 4.0.
 

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Discussion Starter · #9 ·
Thank you, i have a new header for the 4.0, can i use the intake and exhaust manifolds from the 4.0 and bolt them to the 4.2?
 

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If you still have 4.2 head on 4.2, the intake should fit but the exhaust from 4.0 will not. I have talked to a mechanic that had a couple of Jeeps with 4.2 and he switched them to 4.0 injection manifold and said it worked great. I have been wanting to switch my 4.2 head to a H.O. 4.0 head but I have a stainless steel header that I don't want to replace. The reason I wanted to switch was because of horse power gain and I was going to stay carbureted.
 

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Discussion Starter · #11 ·
Thank you, this forum and y'all information is priceless. Thank you. I'm gonna try and find out what water jets need to be plugged in order to put the 4.0 head on the 4.2, so i can use intake and exhaust, with fuel injection. Sounds like a mean combination. Thanks again
 
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