So ive spent the day looking for vacuum leaks and found nothing.

heres what happens:

Started up this morning great, pulled up to the end of my driveway, stopped for traffic and idle was at 700ish. get about 1.5 miles, get to a stop sign and my idle is around 2-300 (first line on the tach) but it doesn't stall or die...is fairly smooth as well. get home, throw it in park and idle is still at 2-300.

A couple hours later I had to go get something and started it up fine, idle was at 700ish but then steadily dropped to 2-300, once again didn't stall or die. and was smooth.

since I had a new IAC laying around from my previous xj I installed it. idle was still 2-300. this was an hour after I got home, started it up, ran at 700ish then again steadily dropped.

 

Should point out its a 1989 4.0 aw4

 

Ever clean the throttle body? A bod O2 sensor can cause th same issue.

Also, if you've never done tis, here's a chance to do some cheap and easy preventative maintenance:

Click on the "pink link" in my signature and perform the procedures in posts 1,2,3,4, and 5.

Also make sure your intake manifold bolts aren't loose.