There could be an exhaust leak near the O2 sensor.
The throttle body was serviced as well, I am assuming. Did you replace the throttle position sensor, (TPS)? It has to be calibrated to manufacturer specs with a multi-meter as well.
Other possibilities are;
A bad ECU
Check to first items on the list and get a Haynes manual if you don't have one.
Tracking down bugs after a complete overhaul can take some time and not always be what you would expect.
Take some time to make sure everything is calibrated and break it in correctly.
Remember, a motor uses more gas when it is brand new and gas mileage will improve as the engine breaks in and things will change with time.
Tuesday, March 15th, 2011 AT 3:01 PM