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.
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.
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