Heres some info related to your specific year model.
On the 86-88 models,the wiring harness to the iac valve has a diode built into it.You should see it a few inches away from the harness connecter that plugs into the iac valve.
The 89-93 models have a diode too,but its built into the iac valve itself.All remanufactured iac valves you buy from the parts store,& most likely Ford too,come with the built in diode.
You need to test your iac valve & the wiring harness to make sure theyre both functioning properly.
Use a multimeter,turn the key on,test the iac harness red wire,it should have 10-12volts.
Now set the meter to ohms & test the other iac harness wire,it should have 10,000+ ohms or Infinite.If it doesnt,the diode is likely bad.
To test the iac valve itself:
1)measure between both of the iac terminals,you should get 7-13 ohms.If not,replace the iac.
2)touch the red meter lead to either of the iac terminals then touch the black lead directly to the body of the iac,take a reading.
Now leave the black lead against the body and move the red lead to the other iac terminal,take a reading.
Both readings should be 10,000+ ohms.If not,replace the iac
If the diode is bad,in your harness, you can either replace the diode or install a remanufactured iac that already haUse diode built into it.
(86-88)
If the diode is good in your harness,but you need to replace the iac valve,you can use any iac valve from the 86-93 model.
(89-93)
You can use any iac valve from the 89-93 model,but you cant use a iac valve from a 86-88 model.
If your harness & iac valve passes the tests above,follow the steps in the link to do a base idle reset.You might wanna check for vacuum leaks too because theyre usually the #1 cause for idle problems.
**Its IMPERATIVE to adjust the idle via the set screw with the iac harness disconnected,so that idle is set between 600-625rpm.If the set screw has idle above this rpm level,the iac will lose its ability to adjust idle via the ecm.The engine is supposed to idle at 600-625rpm via the set screw (mechanically) then the iac is supposed to take over idle control (electrically)from 650rpm & up.The iac/ecm loses its ability to control the idle if this duty cycle is not met.This is what causes the idle to surge/rev up/down.
A dirty iac also causes idle problems & so does loose/corroded connections at the salt & pepper shakers(aka: 10 pin connectors behind upper intake/ white/black)
Summarized/Corrected Base Idle Reset Procedure