1993 GMC Jimmy Straight gas out of tail pipe

My Jimmy has a Vortec, 6 cyl. After getting it from my sister, gas has started to shoot out of the tail pipe. My husband and step son are very mechanically inclined but neither have worked on a vortec and dont know what is causing this. I am trying to learn and would like to keep the truck if the repair isnt to expensive or extensive. Any suggestions as to what it may be or what we should look at? It will start and run, however it tries to die not long after starting it up. Any help would be greatly appreciated. Thank you!