Hey guys, my girlfriend has a 2005 Grand Prix GT. Last summer, the car threw a CEL. I went and had it checked, code told me there was either, 1. a vacuum leak or 2. the gas cap was not on.

I replaced the gas cap, because it def needed to be. I also reset the CEL. After maybe 25-30 miles it popped back on saying the same thing, and then the on board computer told me to "check my gas cap". So I did. It was fine, tight and everything.

Since then the gas mileage has been getting worse and worse. Last summer it was around 25 and now its like 17-18ish. Also, i noticed that when the tank is empty-ish, ill remove the gas cap and there is no pressure change. Which leads me to believe there is a type of vacuum leak somewhere.

Im no stranger to working on cars and what not. Have been for years. You guys think this is something I could diagnose and fix or should I just have her take it to a shop?

Any advice is greatly appreciated.