2008 Chevrolet Impala Fuel System Other Problems

WE OWN 2008 CHEVY IMPALA WITH 40,000 + MILES. THE ENGINE LIGHT GOES ON AND OFF. WE OFTEN CHECK IF THE GAS CAP IS ON TIGHT. THIS USUALLY HELPS. THIS PAST WEEK, ALL WE TRIED WITH GAS CAP DID NOT HELP. WE TOOK CAR TO GM DEALER. THERE IS A DEFECT IN THE GAS CAP. HOWEVER, THE NEW GAS CAP COST US $80 PARTS PLUS LABOR AND COST OF ANALYSIS. WE HAVE OWNED MANY CARS AND NEVER HAD TO REPLACE A GAS CAP. EVEN A CAR WITH 350,000 HAD ORIGINAL GAS CAP. THIS IS UNFAIR FOR OWNER TO BEAR THIS EXPENSE. THANK YOU. *TR

GM ADVERTISES THAT THE 3.9 LITER ENGINE IS SUPPOSED TO GET 28MPG AND WE CAN'T GET MORE THAN 25MPG IN EITHER OF OUR IMPALAS EQUIPPED WITH THIS ENGINE AND BOTH CARS HAVE OVER 10,000 MILES ON THEM WHICH ACCORDING TO THE DEALER MEANS THEY SHOULD BE BROKEN IN. I'VE TALKED TO GM TWICE ABOUT IT AND THEY BLAME THE EPA FOR THE ADVERTISED NUMBERS ON THE WINDOW STICKER SO IT'S NOT THEIR PROBLEM AND THE DEALER DOESN'T KNOW WHAT TO DO BUT THEY HAVE TOLD ME OTHER CUSTOMERS WITH THAT ENGINE HAVE COMPLAINED AS WELL. WHO DOES THE TESTING ON THE CARS TO GET THE FUEL MILEAGE NUMBERS THAT ARE ON THE WINDOW STICKERS? I THOUGHT THE NEW NUMBERS FOR 2008 WERE SUPPOSED TO BE MORE ACCURATE THEN IN THE PAST. WE'VE HAD FOUR OTHER NEW IMPALAS (01,02,04,04) AND THEY ALL GOT THE FUEL MILEAGE ADVERTISED SO WHY DO THE 08'S WITH THE 3.9 DO SO POORLY? *TR