POOR GAS MILEAGE

Hello, evaporative emissions concerns will not cause poor fuel economy. What are you doing to calculate economy? An STS should get probably 14-16 MPG in the city, so you're not really that far off. I would definitely recommend having your engine performance checked to make sure all is as it should be, but perhaps your expectations are too high. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops