My 2005 Roush stage 1 has the Roush exhaust that has a great sound. I noticed the other day that when decelerating the car does not go to full deceleration until about 3 seconds after getting off the gas. The exhaust has a nice rumble at that point.
I had a 2002 GT that I inserted a small washer in the IAC tube that made it go to full deceleration instantly. On that car however the hanging idle was noticeable when shifting and general driving. The 2005 seems to shift perfect with no noticeable hang at all. It would just be nice if it went to the burble as soon as you let off the throttle.
I figured I'd chime in because I had the same issue and it was driving me crazy...The hanging idle made it seem like I didn't know how to shift, and I was also trying to teach my son to drive stick and it wasn't helping. Couple of things that I tried that didn't work, but first a couple details about the car...I bought my 06 GT and it already had an intake, BBK
62mm throttle body and a SCT tuner with previous user tune settings.
#1. I tried to set the idle lower via the tuner hoping the lower idle would help it return to idle faster....that didn't work and it actually caused some idle surge as the car was trying to keep from dying.
#2. I bought a new throttle position sensor (TPS) thinking it was bad and having read that it could be the cause.....nope
#3. (what worked with back story) ... Because 05 and up mustangs use "drive by wire" voltage changes to control the throttle body flutter valve position, there are instances whereby the voltage remains - even at idle or when the clutch is in - at a level that suggests to the throttle body that you are still touching the gas pedal.
If you have modified your mustang or have an upgraded throttle body, odds are there is a voltage set screw on it that regulates the voltage seen or put out by the TPS. If this is the case (it was for me), then watch the video in the link below and get to the part on checking and adjusting TPS voltage [start at 1:15]. According to other forums and threads, a reasonable resting voltage is somewhere between 0.88 and 0.98... at 1 volt and above, the car thinks you've touched the gas pedal or have not fully come off the throttle after depressing the clutch for a shift. Mine was at 1.12 and I adjusted to, and went with 0.98 volts because I wanted the best throttle response possible without the hanging rev. It worked!
If you want to be a bit more conservative so you don't have to repeat the process you could go somewhere in the middle of the voltage range. Keep in mind , the lower the starting voltage, the more pedal you need to prompt a throttle response. (My F-150 must be set at 0.01 because I feel like I have to stop the gas to get a response : / )
Anyway, I hope this helps you or someone else