Need Help With Crazy High Idle

[Christians_4v]

Hey guys, new member here but always reading up on the forums.

So I have a 2004 GT with an 03 Mach 4v swapped in. No crazy mods, just an x pipe, cat back and 4.10s. Car runs great most times but once it runs for about a minute the idle surges for about 15-30 seconds and goes all the way up to 1500-2500 and stays there regardless if I turn it off and back on. I know these 4.6s have that hanging idle till you come to a stop and I can deal with that but this thing never comes down. Well, some days it'll be fine and then some days it idles really high.

Things I've done/cleaned:
New air filter
Cleaned MAF
Cleaned TB(multiple times)
Cleaned IAC/replaced gasket(now has restrictor plate)
Checked for vacuum leaks(smoke machine)
Cleaned all connectors(compressed air)

I'm stumped because sometimes it idles just fine at around 850-900 and then some days it 3x that. Again, I'm not worried about the hanging idle that drops after a few seconds that these 4.6s do, it's the constant high idle that's driving me crazy.

If you guys have any suggestions on what I should do, I would greatly appreciate it.

 

[wmburns]

Normally would asked what have you done to completely rule out any vacuum leaks? But since the intake has been smoked we should assume there aren't any leaks in the intake. However this still leaves:

• PCV valve
• EVAP system and VMV valve. Low tech test. Pinch off the EVAP line. Does this improve the situation?
• Valve covers, oil dip stick?

Which IAC is being used (black vent or non-vented). For the 2004 MY it should be non-vented.

Have you ever attempted to adjust the throttle body idle set screw to change the idle?

Does the throttle move freely and close completely to the SAME spot every time?

Do you have an ODB2 scanner?

Here's some more information on how to trouble shoot.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/