As the title says, i am only getting 12 or 13 mpg, which is rediculous, I just replaced the front O2 sensor, for the second time, but for some reason, i always have code 21, the manual says the sensor voltage to the computer isnt fluctuating? Whats that supposed to mean? Could it be the rear sensor, because i seen a thread saying that he replaced the rear one and it fixed his cel 21?

Also im going to get the fuel filter replaced tomorrow, im sure its the same one on the car for the last 10 years, well at least the last 3 that ive had that car, could that cause real bad gas mileage?

And i checked vacuum today, it read between 13-14 which is low, should be at like 17, and i just replaced all the vac lines with silicone hose, all except the brake booster, ill get to it someday, perhaps its leaking, because it sounds like i can hear a sucking sound coming from the intake manifold, i just cant tell where its coming from. I read that cams lower vac, but would it lower it that much over stock?

Any help is appreciated.