Need a hand: Hanging idle with all new parts

[Overused03]

Hello, I've been tracking down a nasty hanging RPM for a while now and I'm lost. My 2003 4.6 2v New Edge will stick with about 3-4 seconds worth of rpm after letting go of the throttle.
I've done every reset, adjustment, and relearn. Proper TB setup and relearn multiple times and variations and replaced basically every part included in the IAC system. (Multiple times)

New parts: IAC (tried aftermarket and OEM), TPS, larger TB/plenum, hoses and lines, TB cable, intake.

No vac leaks, only part I haven't replaced is the PCV valves

I tried putting a restrictor plate in the IAC like I've seen and read however the car shoots up to 2500 RPM and stays there no matter what.
The IAC sticks open permanently at that point and crushing the tube is the only way to slow it down. Even with it unplugged the IAC stays open when the plate is installed.
If the IAC screw is adjusted to its lowest setting the idle returns to normal and all is well. However once the car is brought to over 2500 RPM again the IAC sticks at that 2500 for a good minute before calming down. Before that happens the hang is gone and the car drives like a dream with a super nice and responsive throttle.

The restrictor plate seems to be the best bet to solve my problem but like I said for some reason it makes the IAC stay open.


TLDR: RPMs linger for too long even with new parts. Tried every trick/reset I know/read. With a restrictor plate it should solve it but the IAC stays stuck open at 2500 RPMs with it in.

Anything helps, thanks!

 

[Mustang5L5]

If you unplug the IAC and drive around, does it still ever hang? What if you cut a small gasket and block the IAC ports internally so that even if the IAC hangs open no air can pass. That would either confirm or eliminate the IAC as a suspect.

Because if the RPM's still hang at 2500 with the IAC blocked, then the issue is elsewhere. Either the TB blade isn't fully closing or you have a vac leak somewhere. A smoke test might be beneficial.

 

[Overused03]

I completely forgot about this thread

I fixed the issue by using a junk IAC and ripping it apart. I found that there is a super small bypass tube that runs inside the valve. At the diaphragm in the valve that tube exits, so I assume that the extra pressure made by the plate was strong enough to pull the diaphragm and keep the valve open.

I put a small dab of glue in the intake hole for the bypass tube in another spare IAC, installed it with the restrictor plate and everything works perfect!

No more idle hang, no more stuck RPMs, and even the IAC adjusts accordingly to A/C and hydroboost draw.

Only remaining "issue" with it now is the cold start doesn't idle high. Meh no more hanging throttle and good decel so I'm happy.