I had the same (or a very similar) problem. I have a thread on it here somewhere, with some suggestions that others left me, if you want to check it out.
Seems like a tremendous number of people with Rangers (and probably BIIs) on here have high initial idle issues. And I concur that the way mine was racing during warm up was definitely
not normal or good for an engine. It was also doing it when it was warm out, so cold was not a factor. Replacing the original Motorcraft IAC with a Hitcachi one from Advance did diddly squat, so I’m not really sold on the IAC fix. Then again, others one here have stated that they went through 3 IAC units before landing on one that fixed the issue.
To me, it seems utterly bizarre that so many of any one part would fail time-and-time again right out of the box (unless they’re all coming from China, in which case, I’m surprised that any ever work). Similarly, I can’t begin to comprehend how all stock and aftermarket Ford pumps would whine, though ppl on here swear they do. I don’t get it. Anyway, sorry to ramble. Just wanted to let you know that you’re not the only one and that a Hitachi IAC isn’t a slam dunk.
My truck has been sitting in the garage all summer and fall, coming out only occasionally to be worked on (I can barely squeeze it in there because the structure is essentially a cinder block shed). Oddly, now that I’m not driving it around, there seems to be no idle issue anymore. Guaranteed, if I finish working on it and put it back on the road, the issue will return. Guaranteed.
Here are the threads I mentioned above, in case there is any advice from others there that’s helpful. As I remember, RonD had some good tips on troubleshooting:
https://therangerstation.com/forums/showthread.php?t=176685
https://therangerstation.com/forums/showthread.php?t=176156