Is the iac only used at idle only? or does it work at a slight throttle position too?

Thanks

99 f250 4x4 auto 256K

 

You sure have a lot of threads opened.
The IAC is opened wide at cold start for the extra air needed with rich fuel injection at that time.
All F.I.gas engines need to do this one way or another.
It remains open beyond the 'normal idle' until,the engine begins to heat up as detected by the cylinder head temperature sensor and intake air sensor.
At hot restart it is opened according to the above mentioned sensors inputs to the computer.
Cold start shows up as high idle then slowly comes down.
After hot running conditions are attained the IAC is modulated by the computer to maintain the 650 RPM idle below about the 1200 rpm +\-.
The idle speed is set in computer program via a table and is non adjustable from the outside.
The throttle 'stop' setting is not to be touched for any reason.
The crank sensor is a Tach pick-up that feeds the CRANK RPM signal back to the computer.
The computer compares the signal to the table then make the rpm correction via the pulses to the IAC until the rpm matches the table value within +\- 50 rpm..
.
At deceleration from road speed with the throttle closed, the IAC closes and fuel injection is greatly reduced to save gas.
This action is Signaled via the closed signal from the TPS and the VSS signal reducing it's speed indication output.
As the engine nears rpm below about 1000, the computer smoothly takes control of the idle again by opening the IAC and adding fuel to match the 650 idle speed..
Thjis action normally is so smooth the driver never feels it.

During 'this specific time', the computer uses it to update variable tables with new values when the computer has the least engine control actions to do.
More than you wanted to know.
It's not just a simple operation.
It's an 'output control sensor' driven by the computer.
Without it you could never get in, cold start the engine and drive away without stalling etc.
Any engine that that has a cold start and drive away issue, has a problem to be addressed.
Good luck.

 

Your right too much to know. Thanks for the post.