Gas Mileage and Thumping Noise

[snoopy1]

I have two problems, the first is gas mileage. For a long time my check engine light had been on, recently I changed the pre-cat O2 sensor. Light went off, gained 50 extra miles to my tank. Every now and then the light would come back on for a little while. I changed the post-cat sensor. I got 80 miles less to the tank. I'm extremely confused. What have I done wrong or what am I missing?

Issue #2: Sometimes while idling, I hear a thumping noise that strongly resembles a subwoofer. It comes from the engine compartment, or so I think. I notice it more while the heater is on. I sometimes hear it when the heaters off though. Can't really give more details on that one.

Can anyone hook me up with some help?

 

[Jspafford]

I am not sure on the mileage issue. Did you pull the codes to see what they were telling you before you replaced parts? If not I would.

The thumping noise is probably the A/C compressor kicking on and off. Sometimes it is loud, and sometimes not. Depends on the truck. The A/C compressor is activated in all but the "floor" setting on your heater controls.