98 Grand Marquis Poor Fuel Economy

Hi All: I noticed recently that my gas mileage on my 98 Grand Marquis has gone down. I used to get 21-22 mpg now I'm dowm to about 17. I
have not changed my driving habits. The engine is not skipping and runs as smooth as ever. Admitteldy, I haven't gone in for a major tune up. Someone suggested spark plugs and O2 sensor. I can problably tackle the plugs although it looks like I will have to remove the fuel rail I have never attempted this before. I have no idea how to change the O2 sensor. Should I try to tuneit myself?
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Have they changed to "clean burning" gasoline recently in Houston? We just did here in the Austin, TX area and I noticed for the first time on a trip to Dallas last weekend that I was getting 17.8 instead of my usual 22-24 mpg on our '92 Grand Marquis. After I filled up in Waco on the way back, I was back to 22.5 MPG for the remainder of the trip driving exactly the same way. I heavily suspect the new shit gas, but will experience a few more tanks before knowing for sure. Stan K.
Finite Guy wrote:

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Stan: I thoght of that issue too. This could be the case. I live in Massachusetts. I also change my tires from Michelin Symmetries (they wore out) to Good Year Aquatreds. Some one told me that an agressive tread pattern can cause a reduction in gas mileage. I'm bettting that the gas could be the culprit.
On Tue, 26 Aug 2003 10:01:15 -0500, Stan Kasperski

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Changing brands of radial ply tires of the same configuration cannot be responsible for the kind of effect you described. It would take something dramatically different to account for the discrepancy of 17 mpg vs 21.5. That's almost a 20% drop. Not sure how you could determine when and if they changed the gasoline formulation, but I would be suspicious if the car is running otherwise normally with no CIL etc. For myself, I make the trip to Dallas and back many times and this was the first time that I didn't get more than 20 mpg even against head winds with the AC on at around 72-74 mph. Last weekend the wind was generally calm and from the side so the biggest factor affecting gas mileage wasn't an issue. I suspect that the new "improved" gasoline hadn't been completely stocked in Waco when I refilled there and resumed historically normal fuel efficiencies for the remainder of the trip. Anyone know how to tell whether your area is using reformulated gasoline? Stan K.
Finite Guy wrote:

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Someone suggested it could be my O2 sensor or spark plugs. Would the car run rough if this were the case? I know that my brother has a 99 Crown Vic and has had to replace three of the coils (the ones on top of spark plugs) but the engine had symptoms of roughness etc. My car runs fine.
On Wed, 27 Aug 2003 09:50:19 -0500, Stan Kasperski

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