My header was cracked when I bought the car and the O2 was reading lean, so the ECU added fuel. It cut down on my performance and gas mileage.I had the header welded at the flange and changed the O2 and the car ran a shit load better, better gas mileage, too.
~LukeFacebook1969 Cougar - 11.91 - 380HP2007 GSXR 7501999 OZ - SOLD - 15.26 NA - 14.71 on 75 shot