What could it be?
My old-ish 99 GMC Sonoma pick up is getting horrible gas mileage. It started having trouble after replacing the fuel pump and fuel line last fall. I got ripped off by the guy who did it so went to someone I trust more to fix the small leak and check everything out. 2 tanks later (three months later...I don't drive this truck all that much) I noticed that I was barely getting 10 mpg. I took it back to my mechanic and he did a full tune up and said to bring it back in after 2 tanks. MPG was fine after tank #1, a little worse after #2 and only 12 MPG on take #3. I'll take it back in next week but I want to know- does anyone have any idea what else could be wrong? It is summer so I have been running the AC more, but should that really make the mpg go down that much? Before last fall, the truck was getting 17-20 mpg.