i have an 88 f150 with the 5.0. i am under the understanding that with the IAC disconnected, the engine should still idle at 750 rpm. to make a long story short, mine doesn't, but if i run the screw in with the iac disconnected, when i recconect it, it idles above 1300rpm. i did this once on my ranger, but i must be missing a bit of that memory.

 

I believe that if you disconnect the iac, it will stay in whatever position it was when it was unplugged. Out of curiosity, why do you want the iac disconnected?

 

I've been told to set idle with IAC disconnected and all your accs on e.g. lights, a/c blower. You also need to adjust your TPS accordingly to your vehicle specs so your ECM knows where idle is.

 

It should be at about 600 rpm no IAC connected and the spout disconnected.
I would start with the timing and get that set correctly that way it isn't messing
with the idle quality. Of course to set the timing you need to unplug the spout
connector and set it to the underhood sticker, I always check the where the timing
is before adjusting it with the spout and w/o spout and compare it to after. Then
unplug the IAC and spout and start it up, adjust the screw to about 800 rpm then back it down to 600 rpm or even 550 then turn it off and press the pedal to the floor
a couple times and restart the truck and see it didn't drift, kill it and hook up all the
connectors and restart it should be idling at 750rpm with nothing on and it should
bump up when the lights, ac etc are on. The area where the screw rides can get
worn out and then the idle drifts to a lower setting not to mention vibration backing
it out over time. a dab of blue locktite or super glue on the side of the screw to prevent it from backing out once you get it set. Don't put it on the screw so it locks the screw and
you will need a torch to loosen it next time

 

i dont mean to steal away from your question, but just to add onto it..

What sensors, if any, have anything to do with idle speed?

 

idle air control motor (iac) for the most part, but also: the throttle position sensor (tps) and air conditioner being on/off can affect it.