Your 86 300zx non turbo starts for like 3 sec then turns off?

The problem in your computer. When the car first starts up the ECU is in open loop mode and everything works(non-controlled by the ecu). Then it goes in to closed-loop mode with everthing being monitored by the ECU. The closed loop circuit of ECU has shorted out. I had the same problem with my 86. To verify this, hook up a inline fuel pressure guage after the fuel filter on the right inner fender well, when you start the car up, have someone watch the fuel pressure and see if it goes away and the cars dies at the same time. If it does the ECU is your problem.