I'm helping a friend with his '99 5.4 Expidition. We think this is an idle air control problem, but not sure. Wanted a second opinion.

The truck seems to run good and idle good, but when idling if you just barely push the throttle down it will almost die...

Truck has 140K miles on it. I'm hoping we can either clean or swap out the IAC and nip this in the bud...

???

 

Possible. Try beating on the IAC with a screwdriver handle or a hammer or something to see if it clears up somewhat.

I'd also inspect the PCV tube running from the passenger's side valve cover to the rear of the intake manifold while you're there.

 

On my '03 4.6L, It runs fine from cold start and anywhere and everywhere you want to go but once it warms up and you kill it, when you try to start it back up it starts but idle speed is way low. Low enough to where you have to rev it slightly and continue to feather the throttle until it will idle on its own but once you rev it up again it wont idle until you repeat the feathering process.
I have a Super chips # 1715 and Air Force I intake kit. I returned the programming to factory and put the OEM Air filtering system back on but the problem remains. No trouble codes show up either.
Does this sound like a potential IAC problem?

 

Closer9, I would get a scanner on that thing before pulling it apart. It is a free service at Auto Zone. I would suspect either a faulty/ near faulty throttle position sensor first. It may also be a crank position sensor going bad. Pulling the codes will tell you.
Kellydown, yours sounds more like the throttle body needs a major cleaning if you are getting no codes. The OBDII systems do not miss a malfunction of a component but a maintenance item like cleaning the TB it sometimes will overlook. I would try that first.

 

More likely a TPS problem.

 

Just wanted to let you know, I took the throttle body off and cleaned it up. There was quiet a bit of varnish built up in there. The results.....you guessed it; PROBLEM SOLVED!!!!!
I have read on here that alot of people have problems with the IAC and EGR causing some problems so after test driving to see if the throttle body cleaning made a difference, I decided to clean those up as well for safe measure. Bottom line Thanks Labnerd.