4.0 Auto hunts for idle or locks/unlocks TC when coasting - JeepForum.com
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Unread 12-07-2013, 08:05 AM   #1
wontacceptthis
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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.

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Unread 12-07-2013, 08:13 AM   #2
CJ7-Tim
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Are all the tune-up parts fresh ?

Quote:
Originally Posted by wontacceptthis View Post
Clean the IAC. Change the o2 sensor before continuing.
Asked and answered.

Test both the O2 sensors and inspect their wire harnesses. Clean the IAC and the throttle body. Add fuel injector cleaner to the next 2 gas tank fill-ups.
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Unread 12-07-2013, 08:47 AM   #3
wontacceptthis
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I haven't done a tune up yet, so I can't accurately say. I plan on changing the downstream oxygen sensor this week, since it is throwing a CEL and needs inspected in January. I've already visually inspected it, but haven't tested the harness for continuity. I will clean the TB and IAC when I get a chance. Thanks.
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Unread 12-08-2013, 04:08 AM   #4
sAe23
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How's the condition / level of the trans fluid?
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Unread 12-26-2013, 04:42 PM   #5
wontacceptthis
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The trans fluid is red(ish) and full.

To update, I still haven't fixed the oxygen sensor, but I cleaned the throttle body and the IAC valve without taking it off. It barely does it at all now. I suspect the seemingly original IAC valve is in need of some more thorough cleaning.
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