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Safety third!
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Have you verified temp sensor voltage/resistance when cold? If it was reading warm all the time it would cause shit idle, stalling, and hesitation on startup because it would think the engine was already at operating temp, and obviously once there thinks everything is great.

Conversely if it was reading cold all the time, it would run super rich because it's stuck in "choke" mode and reverting to cold fueling tables. GM stuff usually reverts to cold when it fails, not sure where the Ford ones tend to go but it's an easy check.

Also verify TPS voltage and adjust as necessary. Not all are created equal and can need a little tweaking.
 

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Safety third!
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11,374 Posts
I know you said the IAC is new but are you 100% sure it is working properly? The symptoms fit. I personally do not know how to test these particular IAC units so hopefully someone who does can chime in.

-Joseph
I've cleaned and tested them by just applying 12V and comparing to a known good one. The plunger moves freely a certain extent when you touch power to it.

The usual symptom I've seen is a fluctuating idle that slowly moves up and down in RPM, almost killing the vehicle on the low end, because the IAC can't compensate fast enough. Usually caused by grunge. My '87 5.0 Mustang was notorious for it and I'd clean it up every 8 months or so.
 
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