Hi. I am a TJ guy learning about YJs. This is a common questions in the TJ forums and I am certain the answer is the same for the YJ:Okay newbie here...how easy is it to change a 4 cyl to a 6 cyl on a 2.0 liter '94 yj?
• frame bosses for motor mounts will have to be cut off and new ones in the correct location will have to be welded on
• different main wiring harnesses
• different radiators
• different computers (my ignorance, here: do YJs have computers? I would think so)
Swapping out your 2.5L for a 4.0L or 4.2L would take enough labor, parts and money that you might as well do a small block Chevy V8 or a Dodge Hemi conversion. For a TJ to do this pretty much the AC compressor and evaporator are the only things you can save. Everything else has to be changed to make a 4.0L work. Many of the issues stem from the emissions gear and how the engine PCM tunes stuff. The parts all must match versions for them to play well together.
If you want to go the GM engine route I highly recommend reading up on it at the Novak site.
Novak Conversions :: Swap Guides
Whatever you end up doing, best of luck to you! If I have posted anything that is bogus I am sure a real YJ person will correct my ignorance. I am just passing on the problems of doing this for a TJ. I cannot imagine it being much different for a YJ - especially the motor mount frame bosses.