Running an LO5 crate motor w/ EBL running an almost factory 350 Caprice tune (tweaked BPC and VE tables slightly, no difference). 65lb injectors, L98 fuel pump throttled down to 13 PSI. New plugs, wires, cap, and rotor. Checked EGR by watching it closely, no movement of the diaphragm when stalling and when I manually apply vacuum it engages and kills the engine instantly.

The car will not idle at all. I have turned the idle speed up to 750 RPM in the PROM and that is the minimum RPM at which it will maintain an idle, but even there, it stalls when I come to a stop or push in the clutch and is almost undrivable. With the idle speed set to 700 RPM or below, it fires right up like a healthy TBI, but as soon as the RPMs come down from the initial startup surge it stalls right out. I see no reason why the car shouldn't idle at 550 RPM. The car drives fine when accelerating and cruising. Currently looking for vacuum leaks, not finding any though. What would you check next?

 

Have you tried resetting the IAC motor. Here's the article from the how-Tos
https://www.thirdgen.org
OOPS trying again
https://www.thirdgen.org/tpimod2/

 

Update: It idles at 550 with the EST disconnected.

 

How's your base timing? What is your idle timing set to in the BIN? Have you verified these settings to be correct? Check your base timing with the EST disconnected and once set correctly, reconnect the EST and see where your idle timing is at. HTH!