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. Ihave 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?