did you remove the manifolds when you did the turbo? If you did, you may have an intake/exhaust leak. Other option is you messed up your O2 sensor by hurting the pigtail wiring (or its not plugged in). If you let me know what you took apart, then maybe we can help a little more. Theres also the chance that there is a crack in the boot from the afm to the turbo or even the j-pipe isn't sealing correctly.