I have a 1989 Chevy C-1500 PU that will not start at all now. It first began to run bad and backfire occasionally. Then it would run for a while then shut off. Seems that after it sat for a while it would run a little farther then shut off again. Then it got to the point that it would start then shut off in 30 seconds. Now it won't start at all. Can anybody tell me what caused this to happen? Thanks for your help.