poor gas milage

when was the last tune up?

I bought it at a Gm dealership. It had a complete tune up . fuel filter ,cap and rotor , new wires , new spark plugs , set to specs ( I pulled them and checked them myself ) new air filter , The lifters were checked and set to specs . The timing was checked and set to specs if needed . I also saw the work order for it . The truck has a throttle body on it . I was wondering if that had something to do with it or one of the sensors . I can normaly find the problem . But this is a stinker .
Thanks .

Usually a sensor will set the check engine light. How bad is the mileage? Have you checked simple things such as vacuum leaks, partially plugged cat converter... As far as the TBI, I believe a 97 is multi-port injection.

I made a trip to Indiana by South Bend I left Murray Ky and I had to fill up at the outskirts of Indinaplis I averaged 16 miles per gallon . I don't remember how many gallons of gas I put in . That was about 320 miles I drove I had another 1997 GMC 4 wheel dr and got 19 to a gallon . I know it can do better . The best I got with my old truck was 20.5 . I will have to check the TBI and see how many ports it has

Honestly, for a GM that is a little lower than one would expect, but not too far out of line for a 350. That may be the best this vehicle gets.