1991 Chevy Cavalier RS

I have a 1991 Chevy Cavalier RS it's a 3.1 stick. That I had traded a camper for and it stoped running after the thired day and we have been puting parts in the car i''ve replaced the injecters, the fuel pump, the sincers, the fuel lines, the fuel rail, the air filter all the fuses. And the car is still not running. I've taken it to a repair shop and thay tell me it's the injecters. Well I put brand new injecters in the car. It trys to start and when it's trying it backfires it's getting gas to the moter but still won't start. Dose someone know what it might be?