just trying to figure what the iac valve does and do I need it? my problem is the idle speed won't come down to 800 while the car is still moving, like when coasting down hill in neutral, I have to come to a complete stop before the idle drops down.
Also I have little or no compression braking for the same reason. I want the idle speed to drop quicker when changing gears, when I push the clutch pedal in.

thanks.

 

This is a very common mustang issue/design flaw. I don't know exactly how the iac works, but I highly recommend doing the iac gasket mod shown in the tutorials section in this forum. I did it and it truly transformed how my car drove! Rpms drop quickly when shifting and there is way more engine braking now.

 

And yes, you do need it. If you unplug it,the engine will stumble or stall

 

The idle air control (IAC) is what allows the engine to idle. When you let of the throttle completely the butterfly in the throttle body closes completely. Without the IAC the car would stall out.

 

again, lots of helpful info, thanks all !!