Oka, so here are the specs
1.No muffler (cut off after cat)
2. Recent Tune up around 2K ago, NGK Plugs and Wires
3. No check engine light
I used to get 15-16 MPGs in the city and 17-18 on the freeway.
Now I get 10-11 MPGs in the city and 12-14 on the freeway..
Is it the O2 Sensor?, Air Filter is still good. Could it be the MAF is clogged up? Recently cleaned out my throttle body and, Ran Injector Cleaner through it but I dnt think injector cleaner really works that well. lol Any Ideas? on what I should do?

 

All out failure of the O2 sensor will produce CEL, but an improperly operating sensor will not. Follow the check procedure in the FSM.

 

is there more than one o2 sensor? where are they at? are they somewhere in the exhaust?

 

on a 3vz there is only one o2 censer, its notorious for being a pain, but have you tried to changed your fule filter? may people have noticed hp and milage to return after a replacement.