It's a '04 Cavalier with close to 50,000 miles on it. It's getting really poor gas mileage and I can't figure it out. My rpm gauge drops down to 500 when I am driving and take it out of gear. Sometimes when my AC is on it raises to 2,000 rpm and will not go down until i turn it off. These are the symptoms of my car. Can anyone advise when it wrong or needs replaced?