here's a little how to and some info from another source
DIRECTIONS:
This is for anyone that doesn't like when the RPM's that are slow to drop or even rise slightly when you push the clutch in to shift. Another symptom, is that when cruising in a higher gear and you let your foot off the gas, the car almost cruises on its own for a bit.
First, understand the problem. The engine is receiving too much air through the IAC valve. From my knowledge Ford did this for emissions reasons.
As a side effect of this fix, when you downshift, the car burbles a lot more, sounds like a carberated engine. You get a little more engine braking effect.
The FIX:
Most older mustangs have an external IAC valve. Meaning that it is easy to find, follow a small air hose (1/2" ID) to a silver cylinder thing on your intake (the IAC solenoid). Once you found it, the older fix was to buy a 1/2" copper pipe cap, drill out a small hole, insert this into the line the restrict air flow through the IAC, and your done.
The Mach has a slightly different setup.
We have an INTERNAL IAC air supply. Meaning that there is no hose leading to it.
To find your IAC valve, first remove your entire shaker assembly from your car.
Next, look between your intake and the firewall, there should be a silver cylinder shaped object with 2 bolts leading to your intake.
Remove the two bolts and move this assembly.
You should see the following, the IAC assembly has two "ports" with the solenoid controlling the air flow between these two ports, what we want to accomplish is to restrict the airflow leading through this.
To do this, there should be a small gasket on your intake, or it could be stuck to the IAC valve.
Remove this (carefully)
Trace out the shape of this gasket as well as the holes for mounting bolts, and the two air holes, onto a piece of 1/32" aluminum or something similar.
Cut out the outline of this "restrictor plate" from the aluminum.
Cut out the holes for your bolts to go through.
Cut out a single one of the Air holes.
Now the tricky part. You have to decide how big to make the other hole because this determines the amount of air you are restricting.
The smallest you should go would be around 5/32", the worst case if you go to small is your car will stall when fix is complete, if this happens just take the plate out, make the hole bigger, and check that.
The hole I used is around 9/32" which is about the medium you would want to go.
After your "restrictor plate" is cut out, mount it with the small hole on the passenger side of the IAC, and make sure to replace the gasket against the intake.
Start your car, you should notice the RPMs drop when you start back to normal very quickly.
If it idles smoothly, try putting turning on the AC, this will load the engine down.
If your car stalls out, remove the plate, make the hole bigger, try again.
If still idling smoothly, bolt your shaker back on, take it for a spin. You might need to adjust your driving/clutching style after being used to the crappy RPM hang, so don't be discouraged.
Also, you might want to disconnect your battery during the procedure to make your computer relearn it tweaks after you finish.
If, after you start the car you notice a "whistling" sound, then take a look at your plate, make sure the BIG hole is at least as big as the hole on the IAC valve. Also make sure edges of holes are rounded (sharp edges are bad), and the holes are round. This happened on mine, and I fixed it.
iac restrictor plate
