2004 Impala Burning a lot of gas

I have a 2004 Impala (3.4l) that is burning a lot of gas. Sometimes there is also a smell of gas in the car. I have read another post on this excellent site that said that the gas smell could be from a faulty fuel pressure regulator. Would this also cause the car to burn more gas (I know this is probably a stupid question). Are there any other causes of bad fuel efficiency that i should be looking for. Also the check engine light has been on in my car for a long time now but my mechanic told me that it was something to do with the gas cap. When the cap was wiped clean and replaced the light went out. Could this have something to do with it. Also when i turn the key it sometimes takes a few seconds for it to start, but its been like this for a long time as well. If i do, however, turn the key ahead for a few seconds and than give the final turn for ignition it works fine.So here is a recap of my cars symptoms:

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Any advice or suggestions would be great.
thanks in advance. Not sure if it is a 6 or 4 cyl.