I have an 01 LS that I recently replaced the plugs on because of some engine vibrations and I figured that since they hadn't been changed in what I think was around 60k I would change them. I did the first 4 myself, all the driver side ones, and then the passenger side back plug ended up being really stuck and since I was leaving for school in a day or two I got a local shop to change it out. I then drove all the way out to school, 1300 miles or so and averaged 27.6 mpg. When I got back everything seemed normal and it was working OK so I changed out the last three and directly after that I noticed a decrease in fuel economy. The issue I have is that I don't know whether that was bad gas, different driving styles from not doing city/town driving in a while or if it was something to do with the plugs. Directly after I noticed that I put some of that fuel treatment/upper cylinder lubricant stuff they always sell at Autozone in and I noticed a small but noticeable increase in fuel efficiency which lead me to believe I got a bad tank of gas. However since then I have not been getting quite the same mileage. Part of it may be the difference in altitude (I live in NJ and go to school in KS) or the difference in gas quality out here. However my main question is this, could a cylinder not be firing or could one not have full power due to a bad connection from the coil or would that be much more noticeable. I get a little bit of a rough idle in park but not nearly as noticeable when stopped in drive. The last thing I could think of would be a new fuel filter but according to the Autozone people, who I would not be suprised if they were wrong, the car has a filter on the pump meaning it is a lifetime part. Either way if anyone has some input into this weird problem I would love to hear it. Thanks.

 

Gas in the midwest , kansas, has a high percentage of ethanol in it. Ethanol has less energy per gallon than straight gasoline. I have always experienced lower gas mileage in the 'corn' belt' My experience only. Hope this helps.

 

That explains the fuel effiency problem, any idea whether it cause a rough idle?