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1990 4.2 TO 4.0 SWAP or 1984 258 rebuilt.

3843 Views 5 Replies 3 Participants Last post by  daddyjeep
Here is the dilemma, I found a guy who has a rebuilt 4.2L with all the fixins (carb/intake/exhaust manifolds/new pumps, water and oil, a cam and forged pistons) which he states is an 84 motor he built for a scrambler. I also have found a Jeep owner with a 4.0L (125K) who is putting in a V8. He is willing to give me the whole kit and caboodle for 1800, including tfer case and tranny. I have thought of all the stuff I am thinking I am going to need to do the swap, but need to know if anyone has changed out a 4.2 for a 4.0 and what problems did they encounter? A few questions on my part, and then I will make the decision on whether or not to go with a 4.0 or rebuilt 4.2L. Realize that I am going with the 4.0 as you all suggested because of the better performance, and that I have read allot about it being the better motor than the carb'd one.

1. 4.2L is from an 84, so will all the parts from the 90 bolt on or am I going to be modifying it to death to get all of it to fit?

2. If I use the 84 motor, should I go with his carb setup, and am I going to have to change out the ECM? If that is the case I am probably going to have to change the wiring harness as well?

3. I am going to try and get all the parts off the 4.0 I/E gas tank, exhaust and whatever else wont work from the 4.2L. If I pull the harness from the firewall, will it mate up to the 90 wiring harness at the firewall?

4. Is there anything else any of you can think of that will help me make the decission, and or anything that i have not thought of?

Thanx for the help
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· Retired Moderator
4,387 Posts
if you can then it might be a good idea to try and get a different tranny as the AX15 is known to be weak...if you do some serious wheeling then i would get a different tranny...but if its mainly street then you should be fine (depending on your tire size)...and if ya go wheelin' then easy on the skinny pedal

· Registered
4,276 Posts
I do not think the wiring will work. You will need the wiring harness and the computer from the 4.0. Since the guy is dropping in a V8 he should be willing to let that stuff go with the motor. The fuel injected motor is definately the way to go unless you can get the 4.2 cheap enough and you can add your own fuel injection. You can get the Howell kit for around $1200.
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