I've searched the forums but didn't find any which seems to match this problem.
My daughter has a 2003 expedition with the 4.6L engine. When she takes her foot off the accelerator the thing takes forever to slow down. For example, at 50mph it will take a mile or more for it the decelerate to 45mph. Oh, and occasionally it will surge badly when she comes to a stop.

I've been messing with this problem for about a year -- she lives a couple hours away so I only have an opportunity to work on infrequently.
The things I've replaced are -- TPS, IAC, EGR, PCV, MAF and plus cleaning the throttle body. This weekend I again cleaned the throttle body, IAC and MAF, replaced the air cleaner and replaced the TPS(under warranty). I will also point out all these parts are non-Motor Craft aftermarket so possibly one of these is a problem.

Any suggestions for what I can test to further diagnose the problem?

 

Could be the new IAC valve is bad.
How high is the RPMs at idle, in Park and in Drive?

 

Sorry I didn't see your question earlier.

Daughter took the truck to the shop today and using their test gear found the IAC bad. Replaced that and all is well now. As I mentioned I replaced it about a year ago trying to track down the problem. Looks like the new one I installed was bad from the getgo as it didn't fix the problem.
As I've read here several times --- use Motor Craft, the aftermarket parts are junk........ Sure seems that way in this case.