I just bought a 2004 Grand Prix and noticed I'm not getting very good gas mileage (18 mpg driving @ 65 mph). The check engine light came on a couple times briefly so I took it to autozone. The reading came up that my thermostat was stuck open and overcooling my engine. I was wondering if the engine being over cooled would affect my fuel economy due to the engine not getting up to temp to properly burn gas. Anybody else have a similar problem or input?