1990 jeep cherokee 4.0. When I start it up it idles at where it should when warm and when its warm it idles at about 300rpm. It is running really rich. It doesnt blow black smoke but is rich black tailpipe. I just put in a new coolant tempature sensor and that didnt do it, not sure if when i did that id i needed to reset the ecm somehow.

 

if you need to reset the ecm take the battery cables off and leave them for a little while. or pull them off and touch them to each other for a couple minutes.

 

Sounds like the IAC is carboned up to me. Try that (Free) then see how it does when warm. If it continues check the new CTS to see if it is well grounded and the wires are not crimped anywhere.