4.0 Auto hunts for idle or locks/unlocks TC when coasting
I'm having a strange issue with a newly acquired 99 xj. Its a 4.0 and a auto. It is throwing two codes related to the downstream oxygen sensor, but it runs and drives perfectly fine otherwise. It starts, idles, shifts, drives normally with one exception. If I let off of the gas, and coast in gear at nearly any speed, the RPMs will drop down to about 800, then rise back up to 1200, and hunt back and forth between 800 to 1200 until you either stop, or accelerate more.
I can't tell if it's the torque converter locking and unlocking, or if it is the idle control motor opening and closing (pushing/ pulling maybe? I haven't looked to see how it's set up on the 4.0 yet)
Clean the IAC? Change the o2 sensor before continuing? Throttle position sensor? Speed sensor? I'm not yet familiar with the AW4 or the jeep fuel injection, so I'm not sure which is the most likely culprit.
Thanks for any help.
stock 99 xj, 200k+ miles :)