Just put on plate and it makes it a joy to drive. It was very annoying to have the rpm being lazy about dropping, and sometimes when you push on the clutch to shift it even would raise some rpm's. Now the engin braking is back like a carbed engin. The gasket is different on the 3.9. I made the plate out of 1/32 like they did on the 4.6 site. For mine, a 7/32 inch hole was just right, I tried 1/4 inch but the idle was still a little high and lazy. Thanks Great Cornholio.

 

Any pics or a link to the process?

 

It started here: https://mustangforums.com/m_547045/tm.htm

For a 3.9L I suspect you could use a gasket as a template for making a IAC restrictor plate. I wimped out and bought a ready-made one for a 4.6L about a year ago