unplug the iac and try this not sure with the xfi system but anyways this is courtesy turbo nasty
Try this.
Car at full operating temp turn off engine and put a jumper on the upper row right 2 term on the adl port.
now with the jumper in place. turn key on NO Start and leave it there. Now go and disconnect the IAC elec connector from the IAC. Leave it off, You may have to take off the hard pipe/duct to get to it.
Remove the little cover over the min air blade setting screw if your car still has it on.
Now go back and turn key OFF and REMOVE jumper.
(For this next step I recommend someone to help you) Now start the car, if it will not stay running try turning the screw in a little at a time until you get it to stay alive. Now adjust the RPM to around 650 rpm in GEAR.
Turn OFF ignition, plug IAC conn back in.
Key on NO start and adjust your tps volts to .45 -.47 @ idle and 4.6 -4.7 WOT.
Now reset the computer (remove power for a few moments).
Now start the car up and see what you have. It may take a few attempts but this works for me and both my GN's and the past 3 others my friends had me tune for them.