Does the car buck and kick also ?
If I were you unless it is less than a couple of years old I would replace the CTS, these do bugger up, and only about £10, can have drastic effect of fuelling as it is the main corrective factor for the CO.
On stage 1(GGR/westfield chip) I was getting at worst around 160 to a tank, I can get over 400 now, after mapping, best money imo that you will ever spend.
tabetha