2.5 to 4.2 swap - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 02-05-2001, 01:34 PM Thread Starter
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2.5 to 4.2 swap

Here's the deal, I got a 91 YJ with the 2.5L that is on its way out. I can get cheap 258 from an 86 CJ or an 87 YJ. First question, is there any difference between the two? The swap seems pretty straight foward, just get motor mounts and other various parts from a pre-90 YJ with the 4.2, right? I'm aware that the AX-5 will have to be swapped out too. Also, what needs to be done to the ignition/electrical system for all this to work? Do I need the harness for the engine too? Right now my Jeep is fuel injected but if I put this 258 in I'll be switching to a carb. Anyone tried this swap already? Emissions is not a concern. Any help would be appreciated.

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post #2 of (permalink) Old 02-05-2001, 01:49 PM
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Re: 2.5 to 4.2 swap

This swap seems backwards, most people go from carb to fuel injected, this swap will give you some problems, Well I havn't done the swap but you are going from something that never really has problems that is something that is swarn at. You will probably pretty much throw away your computer, and you will endup with a computer controled carb on the 258 that is probably one of the most hated carbs, you can make the 258 a nice engine but it takes some work, first the stock carb has to go consider swapping in a mc 2100, quadrajet, or an aftermarket fuel injection such as the one by mopar. Then the igintion system needs to be changed, the techonolygy that's in the current distributor is from ww1, search for igniton upgrades on this board. Also the emissions system on the engine is considered a nightmear, that's why most people swap in a different carb that doesn't rely on miles of vaccume lines and then just run a couple other vaccum lines, usually this help with the engines performance, if you like to tinker and experiment this might be a good swap, other wise wait till you can get your hands on a 4.0

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