The IAC is also used as a virtual dashpot to control emissions when the throttle is cut between shifts. This is normally just a slight annoyance that can be fixed by restricting the air flow through the IAC (more about this below), however if the IAC is operating sluggishly it can be pronounced as your describe.
This is one of the early signs of the IAC growing old, it will still control the idle acceptably however when asked to do more than that it gets sluggish.
You can test for this by unplugging the IAC with the engine an normal operating temperature and idling; the engine speed should immediately
drop and perhaps even stall. If there is any hesitation the IAC is sticking. Though it can be cleaned with rattle can TB cleaner, Ford does not recommend it and more often than not it is a temporary fix.
Also, sometimes the problem is not that the IAC is gunked up internally but that its solenoid "motor" core has become magnetically polarised from years of normal use. This cannot be corrected and the IAC will need to be replaced.
Here is how to fully test the IAC:
- With the engine warm and idling disconnect the IAC electrical connector, the engine rpm should drop significantly or even stall--if it doesn't the IAC is bad buy a new one;
- You can check the IAC with a multi-meter, unplug the electrical connector and measure the resistance between the two terminals, it should be 6Ω to 13Ω--if it isn't the IAC is bad, buy a new one;
- Next check the resistance between each terminal and the IAC body, in both cases it should be better than 10kΩ--if it is less thee IAC is bad, buy a new one;
- Check the voltage between the red wire at the IAC (connector plugged in, key on engine off (KOEO) and ground (the IAC body will work), it should be 12.0V or more--if not you have a wiring problem.
A common mod for years has been to make an IAC gasket having a smaller hole than the IAC output port--this restricts the air flow through the IAC and limits the hanging rpms between shifts. It is also a PITA to do as the size of the hole has to be largely determined by experimentation and each change require removal and reinstallation of the IAC.
A better way is to drill a hole (start out at 9/32") in a 1/2" copper pipe cap and then insert the cap into the IAC feed tube:
By using this method the IAC does not have to be removed, making the trial and error process easy. Try the 9/32" hole and if you find that too aggressive (the engine may stall, or idle too low) open the hole up a bit. I have found 19/64" to be optimal for most engines.
As I have a small metal lathe I made up a small restrictor of aluminum bar stock:
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq