Stalls at idle no fluxuation in voltage at iac

Did you change the ECM to match the newer engine?

https://www.google.com/search?q=1992+GMC+Sierra+ST%2C+5.7+TBI+runs+rough

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Its located right on the throttle body. If your talking about the air intake temp sensor, thats mounted by the air filter or in the plastic tube that goes off the throttle body to the air box, its a 2 wire hook up. The idle air control "iac" is located on the throttle body side. "iac" idle air control. "ait" is air intake temp. 2 totally diff sensors.

idle air control (IAC) or idle control valve (ICV) same part, sometimes listed under one or the other when parts store looks them up. if veh has throttle body is located on passenger side of throttle body (rear side)

did you clean the port before you replaced the iac valve, and measure the pntle before putting it back in, whenyou buy a new iac valve you can't assume the pintle(needle) is correctly placed. the pintle shldnt exceed 1 1/8 inch past the body seat, it can be pshed in by hand, then after it's pu t back in cycle ignition on for 10 second (do not start) then turn ign off for atleast 10 seconds, this allows the iac valve to reset. then start engine.

If the PCM receives a signal that the coolant sensor is cold then the idle will be high.
IAC valve could be stuck open and not retracting. Check signal at 4 wires(open circuit). If ok change the IAC valve.
Warning; don't try to retract the pintle by hand.

Hope it helps.

If you are talking the IAC, Idle Air Control, it's on the throttle body. It has a 4 wire connector on it.

see if you can pull the codes for vehicle ,,but it sounds like a idle air control valve,iac,,,or sometimes trottle position sensor,,,most the time you can take iac off clean part and trottle body

The IAC is not a sensor. It is a stepper motor controled by the PCM. It is sometimes located on the side of the throttle body.

IAC - idle air control valve problem maybe .
Idle Air Control Valve
GM Idle Air Control IAC Function Diagnostics and Repair
007 Idle Speed Problems Scan Data for Idle

check for codes. If no codes are stored then maybe the idle air control valve is bad, or the air cleaner clamps are loose.. If the clamps were loose or a vacuum hose fell off you should have a code stored. If the IAC is bad it usually will not set a code.