IAC relearn
Our 99 XJ does not get driven much anymore, Sometimes it sits for a week at a time without being started. Recently it would not start unless the throttle was held open. Then it would not idle. It would stall as soon as you let off the gas.
I hooked up my Altell scan tool and started looking at the IAC values. They seemed a bit off for what I respected. But not having any history on this Jeep I just watched them for a while.
I began by holding open the throttle just enough to keep it from stalling. The target RPM was 740 and I held it around 650 or so. Slowly the IAC value began to adjust. At first, it seemed to go the wrong way but eventually, it began incrementing, and soon the IAC was controlling the idle speed.
I tested by restarting without touching the gas pedal. I also put the Jeep in drive and reverse and the idle speed held fine. I switched on the AC as well and the IAC continued to control the idle.
I have no idea what the IAC lost its memory, but just holding the throttle open enough to keep it running did the trick to reset. I could have done this without a scan tool but by monitoring, I could see when the IAC actually began to regain control and I knew when to let it go on its own.