1991 Chevy Lumina 1991 Chevy Lumina electrical problem?

How did you read the codes? You should have received a code 12 if nothing else. What's the litre size of your engine?

When it refuses to start, does it crank ok? If so, have a helper crank it, while you check for spark at the plugs and injector pulse.

Did you check fuel pressure, before you replaced anything? Some of the discount autostores will sell a fuel pressure gage for less than the parts you've already replaced.

Does it die at idle or only in gear?

Pressing the schrader valve to see if gas squirts up, isn't the best way. You would have an initial prime when you turn on the key, no crank. You need to see if the fuel pump is running, with the engine cranking. You can turn the key on, no crank, leave the key in the on position, wrap something over the schrader valve, so gas doesn't go everywhere, bleed the pressure off, then, without recycling the key, crank the engine for a few seconds, recheck the schrader valve to see if there is any pressure. You have to do the testing when the vehicle refuses to start. My first thought was, you may be losing the rpm signal. With the engine cranking if there is no rpm signal, you will have no spark at the plugs, no injector pulse. Other than the initial prime, the pump won't run, unless there is an o/p switch in the pump circuit. I don't believe you have that.