I realize this is quite late, but, if I'm not mistaken, the 93-95 or so 4.3L v6 Vortecs have what is refereed to a spider fuel injection assembly. These have a single injector feeding 6 poppet valves via tubes. It is all encased within the split intake plenum. As you take the upper half of you intake plenum off you will find the spider assembly. On one side is the fuel supply/return lines and the injector. On the other side of the assembly is the fuel pressure regulator.
I have a 93 Jimmy and am having a similar problem to yours. I have completely torn into the intake and replaced the entire spider assembly (regulator and all), replaced the following as well: fuel pump, fuel filter, almost all the sensors and connectors except the oil pressure sending unit which seems to be working fine (gauge works as it should). I even replaced the distributor just because my brother had it jury-rigged. New plugs and wires.
I did some research into checking the signal to the injector. I'm not sure if I checked it right or not. From what I gathered, I should have been getting a spike of voltage twice per rev of the motor. Well, I was getting constant voltage as near as I could tell testing on the A side I believe. I'm guessing the other side goes to ground or eventually to ground. I'm not real bright when it comes to these things.
Also I pulled the poppets out individually to check if they were spraying fuel while cranking the motor. NOTHING! The last time I checked, I was getting plenty of pressure. The only thing I know to do that I haven't done is change out the ecm. Is there anything else that could be the problem or could this be my Achilles'?
Tuesday, June 11th, 2013 AT 2:29 PM