Had issues here's what I found been told had a similar problem with idle hanging high, also using a GM style idle stepper motor that was part of a holley 1000cfm throttle body MPEFI chev 383 set-up using closed loop idle control. It was a few years ago but from memory, here is how I licked it: 1. Firstly you have to identify the stepper values that equate to the IAC valve being fully closed. Do this by switching to open loop and physically looking at the IAC valve head while it is installed in the throttle body (car not running) and keep raising the stepper value in the table until the IAC no longer visibly moves any further closed. Then back off 1 point, then set that as your max clamp value. That means the IAC will never be instructed by the ECU to go beyond fully closed and will therefore never get out of kilter with the ECU. To get a decent min clamp value, still in open loop, with a warm engine set the IAC far enough open to give you around 2500 rpm and note that value - set min clamp at that value. Set TP Lockout to around 1% and RPM lockout to around 300-400. 2. Still in open loop, with the engine warm, set the stepper motor value to the max clamp level so the IAC is closed, and adjust the throttle stop so that the engine is idling about 100-150 rpm below your target idle speed. Then do a TPS calibration and ensure that the TP position goes to 0% when the throttle is fully closed. 3. Still in open loop, with the engine completely cold, fire it up from cold and adjust the stepper motor values in the table for each temperature cell so that under 60C engine temp, you are idling around 150-200 RPM slower than your target closed loop idle speed for that temp, and above 60C you are idling around 100 rpm slower than your target RPM closed loop idle speed for that temp. Basically create a set of open loop stepper motor values that have the car idling quite a bit slower than desired at all temperatures. The IAC should be very near the max clamp value with the engine at operating temp. This set-up process is important and it's the key to success. 4. Switch back to closed loop and fine tune tune from there. If it still hangs high, try increasing stepper motor value slightly, or increasing RPM lock-out slightly, or both. If it drops under target and then recovers or starts to hunt, drop stepper motor value slightly. The reason the set-up approach in 3 is important is that when you set control to closed loop, when the throttle goes under 1% and RPM falls under the RPM lockout value (like when you coast to the lights), the ECU implements a ramping strategy that you have no control over. To smoothly drop RPM down to target the ECU subtracts a fixed non-user-adjustable number of steps off the stepper motor table value for about a second, and then adopts the table value for about a second, and then adopts closed loop control. If you have a stepper motor table value too low to start with, at the first stage of this ramping strategy the rpm will hold or rise outside the rpm lockout point again and the car will hang above this point or hunt around it. The reason my car was prone to this is that the GM stepper motor flows shitloads of air and the non-user-adjustable number of steps that the ECU took off the table value at the first stage of this ramp back to closed loop actually increased my idle speed rather than gently guiding it down. The bandaid approach was basically to have stepper motor values in the table that caused the IAC to be more closed during the non-controllable part of the ramping strategy than it otherwise would be. Idling when warm in closed loop my stepper values would generally be 5-8 steps higher than the base value in the table. Let me know how you go. My Chev was boosted too - I don't think your issue will be boost related. Hope this helps. Inserted a screen shot of my idle set-up, done using this approach. Your stepper values won't be the same, but you get the idea.