I have a 1998 XJ. I'm in the market for a used 4.0 I understand that only certain years from certain models will fit my jeep's motor mounts. I believe it's 96-98. But I have a lead on a 1999 Commanche engine and I'm not sure if it will bolt straight up to my motor mounts and accessories. I've searched but there's not much info on the MJ. Please help!

Btw I've been lurking for awhile and finally found a good reason to join

 

Welcome. There's a broader spectrum of engines that will fit than what you posted.

XJ Cherokee and ZJ Grand Cherokee 4.0L engine blocks interchange.

2000+ TJ Wrangler and WJ Grand Cherokee 4.0L engine blocks interchange.

XJ/ZJ blocks and 2000+ TJ/WJ blocks do not interchange without significant modifications. (1991 to 1995 YJ and 1997-1999 4.0L TJ Wrangler blocks will interchange in XJ/ZJ and visa-versa).

TJ/WJ 4.0L Engine blocks underwent clean sheet design changes effective in the 1999 WJ Grand and 2000 TJ Wrangler. These blocks are not interchangeable with XJ/ZJ engine blocks. The reason is motor mount bolt holes and belt driven accessory mounting bolt holes are in different locations, or not present at all, TJ/WJ vs. XJ/ZJ.


XJ: "Regular" (not Grand) Cherokees 84-01
ZJ: Grand Cherokee 93-98 (Gen1)
WJ: Grand Cherokee 99-04 (Gen2)
YJ: Wrangler 87-95 (Gen1)
TJ: Wrangler 97-06 (Gen2)

 

most differences are with the harness clips and sensors. as for renix engine, you should swap head, intake and exhaust manifolds along with sensors and you should be good.

the reason you're not finding much on the 1999 comanche is because they didn't make a 99. production stopped in 92.
first year, 1986 was amc with 2.8L V6 then 87-90 was amc with renix 4.0L straight 6 and 91-92 was chrysler with 4.0L high output non renix straight 6. essentially the same engine with a few differences that is pointed out above.

i know a guy that put a wj block into a 96 xj, so it can be done without too much work.