FiTech EFI system
I'm not using timing control. What are your numbers?
OK, when using timing control the computer will vary the timing, IAC, and fuel all at once and it's tough to get it dialed in.
My loop rate up/down are both set to 15.
Also check under the fan settings and ensure they're DISABLED if you're not using a fan. You have to use the joystick to click left/right to disable it (took me a while to figure out). Otherwise when the computer tries to turn on the (nonexistant) fan, the idle will raise to compensate.
Then under 'closed loop AFR' my 'idle fuel learn rate' had to be tweaked way down, to about 5. Once it settled in and I had my idle screw set (to keep the IAC close to my 5~15 goal) when fully warmed, I pushed the idle fuel learn rate down to 1. Before I did this, the AFR would swing from 11.5~15.5 and the RPM would surge, the IAC would chase it while the AFRs were cycling.. it was a little bit of a mess. Once I turned the learn rate down and played with the idle trim rates and loop rates, the IAC became far more stable and I was able to dial it in.
Data logging is your friend. Just watching the numbers bounce around is fun, but you wind up chasing your tail. Being able to look at the data after the fact really lets you see what's going on, as the out will list the fuel learn, trim values, IAC, rpm, etc.
Another thing that gave me fits is that I had a plug wire that was arcing to ground. The computer was picking it up as a signal and so the rpm readings were FUBAR and the IAC was trying to compensate for an rpm issue that didn't exist. Check your fault codes and see if 'RPM NOISE' is listed if changing the loop rate doesn't help.