I have a 1991 4.0 v6 XJ with a bad motor. My buddy has a motor out of a 1998. Will I be able to just pull out the blow engine from the 91 and install the 98 without the wire harness from the 98?

 

https://www.cherokeeforum.com/f2/che...bility-236512/

You will probably be fine, though you'll have to relocate your temp sending unit.

 

Where do i relocate the temp sending unit to and how?

 

Most folks run it to the t-stat housing. It's basically, drill and tap the t-stat housing to take the sending unit, screw the existing sending unit in (with thread sealer to prevent leaks) and then cut the connector off of the original location, splice in an extension wire and hook it back up. There are several youtube videos that detail this process.
You lose the precision of the original location (it won't read the temp until the t-stat opens) but once the engine warms up it will work as it did previously.

If it were me, I'd get new t-stat housing (they're surprisingly cheap and stocked at the local parts retailers) and do the mod at the workbench rather than on the vehicle. That way if you mess it up or crack the housing you can get another one and try again without spilling a bunch of antifreeze and you also don't have to worry about metal shavings getting into the engine.

The alternative is to drill and tap the head on the 98... this seems overly complicated and risky and 1000x more work.

 

Couldn't you just use the 98 thermostat housing that already has the sensor in it.

 

The 97+ uses the CTS to operate the gauge. 96 and earlier uses the sending unit. The t-stat housing is actually exactly the same for 91-01.

 

Whats the CTS? My 97 uses the sensor in the thermostat housing for the gauge, I know because the gauge will act up when the plug gets loose.

 

CTS - Coolant temp sensor

It's the sensor in the t-stat housing.

For 97+, the gauge uses the CTS (as you mention), but the CTS is primarily an engine control sensor. 96 and earlier also have a CTS in the same location, but also have the separate sending unit at the back of the head.

In order to use a 98 engine in a 91, you use the 91 CTS since it's compatible with the 91 PCM. However, since there's no port for the 91 sending unit in the head, you have to relocate it.
I suppose you could port all the electrical stuff from the 98 and use the 98 PCM and CTS but I think that's probably a whole lot more work than relocating the 91 sending unit.