Well my guess would be that since you installed the header and don't have a tune the ECU is trying to compensate.
Do you have a wide band installed? You could see if the engine was just dumping fuel at startup. I would recommend getting a wideband now that you have a header. It would also be needed for remote tuning if you went down that road.
2012 Fusion SE 3.0L V6
'86 XR4Ti ; 2.3L Turbo, 15psi
1988 F150 Custom
'07 ST ; formally bewst'd - R.I.P
D23 Crew: #07