I recently bought a 93 Ranger 3.0 automatic, It was/is running, but rough. It was especially hard to start when cold. The PO had replaced a water pump (new), fuel pump (not sure if new), TPS (junkyard), MAF (junkyard), IAC (junkyard) and there were a few spare fuel injectors in a can in the truck. Obviously, he had done things with the fuel injectors. I bought a code reader and have seen many different codes as I replace parts and recheck codes. I've bought new (but not OEM) MAF, Temp sensor, EGR, DPFE, TPS, IAC, air cleaner (there was none!). I've really gotten places with these replacements, but things are not quite right. When I am driving it sometimes feels like cruise control is on (truck does NOT have cruise) and the truck will drive along at 45-50 without touching the gas. When I come to a stop, it will act like you all of a sudden removed your foot from the gas pedal and it stops fine. I've been cruising along at 45 with my foot Off the pedal, moved the truck into neutral and could hear that the motor was revving. after a moment, I can hear the "foot come off the pedal" and the idle backs down. When I put it back into drive and accelerate, after a moment or two, it holds the acceleration - like cruise control. I stopped on the side of the road, the truck idled down and I disconnected my IAC and it made no difference. BUT, with it disconnected the throttle sticking stopped.
I believe I have an air leak and thats why disconnecting the IAC has no effect on the idling truck? I've sprayed carb cleaner looking for a vacuum leak, but do not get any surges.
I know the rubber seal at the large plastic air intake hose is missing, but I don't think that would be an issue (the intake between the MAF and the throttle body)
Maybe a vacuum leak under the intake plenum? Maybe o-rings at the injectors?

 

There is a screw on the throttle that is sometime used incorrectly to adjust idle rpm. I don't know if the 3.0 has this screw but take a look to see if it does. If you find it try to turn it back to see if you can change the idle rpm. If so it is what is controlling idle.

Non OEM parts many times do not work well with Rangers. It sounds like your current IAC is not working as it should. I would try to replace it with a new OEM unit.

 

So, I removed the brand new Duralast IAC, installed the old Ford branded IAC and all of my problems went away!!!
Lesson learned, when it comes to vehicle electronics or sensors, use what the dealer would use. OEM parts