Joth

I was checking the voltage off my 02 sensor, right at the ECU to verify it was working, im chasing some heavy rich running, the voltage swings between under .1v right up to .8,.9v, instead of balancing in small variation around the center. At idle itll go from one extreme to the other over about 2 seconds.
Does this heavy swinging voltage range seem normal ? My fuel consumption is so bad (about 12 mpg) that I feel the ECU is not even running closed loop, but running WOT full rich. I dont have any check engine light issues, and the car does run fine, starting, idling and running, no problems.
Ive got a new 02 sensor, new spark plugs, what else can i check, clean the injectors?
If the A/F mapping was stuck in closed loop mode what on earth could I do about it?