Poor Gas Mileage

I have a 1995 Lumina with a 3.1 and I am getting really bad gas mileage around 12 mpg. I have asked several mechanics and they have said that is the O2 sensor. I have replaced this and it did improve the mpg slightly but it is still not the 29 it used to get. I have also replaced the knock sensor, and the throttle posistion sensor. What else could cause this drastic loss in gas mileage?

Thank you,

Did this happen all at once or gradually? How many miles on engine? Check for proper EGR operation, vacuum leaks, PCV valve operation, & have entire system checked with scanner. Almost impossible to diagnose w/o vehicle in front of us.

Did this happen all at once or gradually? How many miles on engine? Check for proper EGR operation, vacuum leaks, PCV valve operation, & have entire system checked with scanner. Almost impossible to diagnose w/o vehicle in front of us.

x2

hows the air filter look?? i was getting a gradual decrease in mileage untill i found a mouse nest almost covering the ENTIRE air intake filter.:shakehead after i cleaned it... the mileage came back.. just a thought.. also make sure you dont have a brake caliper hanging up, that could cause a drag on the motor and it's not as obvious as most people think it is.... GOOD LUCK!