02 impala gas mileage

I just replaced fuel filter and air filter. I drove 317mi on 13.8 gal for 23mpg. used to be 30mpg (to work and back). Cat was replaced at 70,000. currently has 100k. 3.8 liter engine. haven't done plugs,wires or oxygen sensor yet. Car has good acceleration except in overdrive . Its very sluggish. Even the overhead monitor has changed. It used to read that I was getting 33mpg now it says 28mpg. It always was a little high. Also havent replaced the 3 coil packs. My 3.1Liter buick regal used to buck like bull when it went into overdrive when the coils were bad. Any suggestions? Always appreciate your responses! thanks.

I have the 3.8 also, and get 23-25 mpg pretty consistently. However, I just took a 1200 mile road trip and averaged 30.6 mpg on that trip, almost all interstate and cruise control at about 75mph.
Maybe your state has a winter gas blend that is hurting your mileage, or your tires are low due to the cold.

It's about time to change the plugs. Generally at that mileage, the plug tips will be worn and the gap will also be larger than it should. Changing them should give you increased power and better gas mileage.

Might want to check to be certain your throttlebody is clean butterfly and intake horn. check carefully for vacuum lines cracked or dry rot...I also agree with the previous regarding the plugs and a good time to check plug wire resistance. Using the same pump at fill-ups ? I wouldn't be too concerned about the o2 sensor just yet nor coil packs for that matter. Alternator could be running under load more often if battery is getting old and weaker.

thanks for the response '57. battery is only 2yrs old. I had the alternator checked and its still in good shape. right now I'm workin on getting the plugs out. doin the rear first. got 2 and 4 out. 4's boot came off in 2 pieces so I'm doin wires too. I'm going to have to remove the o2 sensor to get #6. Should I just replace the o2 sensor or have it checked and can it be checked off the car? How do I check the throttle body?

I replaced the plugs and wires and had to remove the oxygen sensor to get to plug 6. what would indicate a bad sensor. the tip is a reddish color like the color of bricks. there were no deposits at all which is a good thing I guess. Anyone know what they should look like. Again I have 100,000mi on it. thanks for any input.

The O2 sensor is monitored by the PCM, usually a bad one will throw a check engine light.
Normally, an O2 sensor will last 100K miles, but if you have it off, it will only cost about $100 for a new one. Personally, I don't think yours is bad.
To check the throttle body, remove the rubber corrugated intake hose that is between the fitler box and the engine. Then use a small screwdriver to remove the snap ring holding in the metal mesh. Remove the mesh, and look, you will see the throttle plate. You might need a flashlight.

thanks I-Fly. I did check the throttle body and it was clean as a whistle. I recently took a trip to Rhode Island and got 26.5 mpg . mostly highway miles doing 65-70. Dont really know what to check or change next. I'll wait till spring when the 10% ethanol gas it outta here and see how the mileage is then. Thanks again for your response.

Another thing is to make sure that your defroster isn't on, because as soon as you turn the knob even to the defrost/foot combo, the a/c kicks in, wanted or not, unless the outside temp is below 35 degrees.