1989 Nissan Truck 1989 nissan 4x4 hardbody pickup

Engine Mechanical problem
1989 Nissan Truck 4 cyl Four Wheel Drive 153000 miles

I recently bought a nissan 4x4 pickup from a friend of mine, and am at the end of my rope trying to get it running. It has 153000 miles on the truck but only 18000 on the motor. It is the z 24 motor with the 8 plugs. when he had the truck it started missing and quit running. He let it sit for a couple of years and finally i got him to sell it. When i got the truck it would start and idle but thats it. when you give it gas it died. So i let my mechanic work on it , starting first by replacing the rusted gas tank, fuel pump, battery, air filter, fuel filter, starter, and the bushings in front end. This still did not solve the problem and it would not run, but will idle a little bit, so evidently it is getting fire. It is getting gas to the injector, and the mechanic seems to think the injector is fine. ???? so next i bought a refurbished computer from advance auto and had it put on...still nothin changes????? So please help.. what should be my next step, this project has gone extremely close to budget and still the truck doesnt run.. what can i do?? jp