2002 4.0 auto trans with 98k miles. warm engine idles what i consider normal about 750 rpm approx rock steady with trans in gear. when i put the gear selector
in either neutral or park rpms go way down.. i guess to about 500? almost to the point of engine stalling and then recovers to the normal 750 reasonably
quickly. this is something that just started happening... any thoughts as to why? the battery was disconnected recently.

 

750-800 warm idle for 4.0l with automatic is normal, the drop and recovery is not

The DTR(digital transmission range) sensor tells the computer when trans is in Park or Neutral and when its "in gear", computer uses that signal to sent idle RPMs, computer bumps up RPMs when you are in gear(under load), then reduces RPMs when in Park, using the IAC Valve

Reads like maybe the IAC Valve is sticking when computer closes it a bit(to lower RPMs) when you shift to Park, then has to try and recover, opening it more, to get idle back up

Try testing it in Neutral instead of Park to see if same drop in RPM occurs, if it seems OK in Neutral then maybe column shifter is loose, common on Ford trucks, and computer is not getting correct signal for Park

In any case the IAC Valve is what computer uses to set the idle RPMs so maybe give it a clean and see how that works

 

thanks ron. yes the same rpm drop happens when shifting from say drive to neutral.
need to use throttle body spray on the iac valve to clean right? ie. not carb cleaner?

 

Carb cleaner would work but throttle body clean would be better to use

Thread here with pictures: https://www.explorerforum.com/forums...-w-pics.84220/

After taking the IAC valve off, turn on the key and then plug IAC valves wires in, valve inside should open all the way(for startup) it only moves 3/8" or so
Then unplug the wires and it should close
Repeat that a few times to see if it might be sticking

IAC Valve is a solenoid, it gets 12v with key on, the computer pulses the Ground wire, to set lower voltage when engine is running, and computer remembers/learns the pulse count for say 750rpm, and 800, and cold engine 1,100, and can relearn if things change over time, i.e. small vacuum leaks
And this method is accurate +/-3 RPM or so very accurate way to set idle RPMs
Computer has "target" idle RPMs in memory, to match up with engine coolant temp, and AC ON/OFF, and In gear/in neutral RPMs