Hey, I have a 1994 ford ranger with the 4.0 it idles at 1500-1700 when it reaches operating temp does not do this when cold. When I unplug the IAC the idle drops to normal would this just be a bad IAC or could there be something else causing this?
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Thanks.

 

With IAC Valve unplug RPMs should drop below "normal", to 500rpms, so barely running, or engine should stall, but its shouldn't "idle normal"

With cold start, idle should be 1,100-1,200rpms
Then slowly drop as engine warms up over next 5 minutes
This is Choke Mode
650rpms warm idle for manual trans
750rpms for automatics

Because the idle RPMs do change when unplugging the IAC valve that usually means it is working, not stuck
Could be ECT(engine coolant temp) sensor is showing engine is over heating, that can cause high idle for better cooling, but long shot

Vacuum leak............MAYBE(big maybe), but that should effect all engine temp idles, and computer should close IAC valve if RPMs are higher than they should be, so unplugging IAC wouldn't change RPMs

Unplug TPS(throttle sensor) see if that matters, also see if CEL(check engine light) comes on, it should, its a sanity test of computer, no CEL means computer is not monitoring that sensor and it should, turn off engine and plug TPS back in, restart and CEL should go off
Unplug MAF sensor as well, again should get CEL should come on, and then go off after its plugged back in

It reads like it could be a computer issue
I had to replace the 3 capacitors in my 1994 Computer last June, because of odd running issues
The 3 blue capacitors seen here: Ford EEC-IV

Under $5 for parts, if you can solder you can do it, or any electronics repair shop can, i.e. TV and Radio repair