1999 Chevy Blazer Bad Mileage

Hi; I'm hoping someone out there may be able to help me. Evidently if you don't pay you don't get an answer from the 2carpros. Anyway. My 99' Blazer gets very poor mileage. Has a new tune-up and once it starts it runs great. Problem is the last owner put an inexpensive fuel pump in and it doesn't seem to push enough pressure to the fuel injectors. I realize I'll have to change this, although do you think this could be the cause of the poor mileage? I don't want to get rid of it and give someone else the headache. Could the oxygen sensors have something to do with this? Thanks in advance for any advice that might help. Happy Holidays!