im just curious as to know what are the differences of the HO 4.0 and the 1990 and below 4.0. i have a 1990 and its really hard to find aftermarket parts for it. everything is for 1991 and up, so what are the real differences?

they sell parts like headers and valve covers and throttle bodies for the 1991 and up. but i didnt think the engines were that different. i can hardly find anything like that for the non HO version

IIRC, the difference was about 15HP, and maybe the head was redesigned.

Try http://www.xj-armor.com/ for throttle bodies, spacers, intakes and various stuff for early XJ's (btw, I've heard some complaints about these folks but I have bought several times from them with no problems; headers, lift kits are fairly easy to find but quality ones aren't cheap (found mine on eBay). It's out there, just takes a bit more looking and is generally more expensive.

The 4.0 engine was made into the HO version in 1991 with changes to the head, camshaft profile, and the block castings.
In 1996, the 4.0 L got lighter pistons, a new head with better exhaust flow, a new exhaust manifold and steel valve covers.
In 1997, the block was tweaked for new a oil filter mounting for production compatiblity for the Grand Cherkokee.
In 1999, other block changes were made to the block, camshaft, rockers and head.
In 2006, production for the 4.0 L will be stopped with the 06 Wrangler.