Did you unplug the battery for 20 minutes or so after changing the IAC? If not then I would try that, which will force the PCM to relearn the fuel trims. The PCM has adapted the fuel trims to the bad PCM to keep the car running and now with the new one the air/fuel tables are going to be off. Disconnecting the battery will reset this and the PCM will relearn the fuel trims.
EDIT: Here is the procedure that was in my 06 owners manual which should be similar to yours.:
Because your vehicle’s engine is electronically controlled by a computer,
some control conditions are maintained by power from the battery. When
the battery is disconnected or a new battery is installed, the engine must
relearn its idle and fuel trim strategy for optimum driveability and
performance. To begin this process:
1. With the vehicle at a complete stop, set the parking brake.
2. Put the gearshift lever in P (Park), turn off all accessories and start
the engine.
3. Run the engine until it reaches normal operating temperature.
4. Allow the engine to idle for at least one minute.
5. Turn the A/C on and allow the engine to idle for at least one minute.
6. Drive the vehicle to complete the relearning process.
• The vehicle may need to be driven to relearn the idle and fuel trim
strategy.
• If you do not allow the engine to relearn its idle trim, the idle
quality of your vehicle may be adversely affected until the idle
trim is eventually relearned.
