Looked around here and did not find. My new to me 87 4 x 4, manual shift 2.9 ranger. I have been building on this truck for a while. I bought it for 300 with a blown head gasket. I have a dead 86 2 x 4, automatic 2.9 ext cab that I am robbing all the parts off of for the 87. I did an "old school" valve job "hand lapped" and put these heads on my new truck. This issue was there before the valve/ head gasket job. The truck does not high idle when in "open loop", start up. Once it wams up enough it will idle. My question is, when it is in open loop does the IAC control the cold start idle?
The IAC(idle air control) valve controls the idle at all times, cold or warm.
Fuel injected engines do not have "jets" like a carb, and idle needs to be constantly adjusted, so a "controlled vacuum leak" was used, the IAC valve.
Computer gets RPM of engine from CKP(crank position) sensor OR in your model year the TFI distributor sensor.
Computer has preset idle RPM values that are based on engine temperature.
Engine temp is read from the ECT(engine coolant temp) sensor, this is a TWO wire sensor.
Dash board gauge uses a ONE wire Sender.
Computer sends ECT sensor 5volts, and then reads the voltage coming back thru sensor, if engine is cold(60degF) computer will get back 3volts, as engine warms up voltage coming back will go down, under 1 volt is warmed up.
So on cold start RPM may be 1,100, then drop to 900 after a minute or two then drop to 750 after 5 minutes(warmed up).
IAC valve reads like it is not working at all in your case, because on cold or warm starts RPMs should go up to 1,500 then drop quickly to preset values, this is the computer open IAC valve all the way then closing it.
Often pulling it off and cleaning will get it working again.