Turns over, backfires out of carb, but won't start on 1990 Chevrolet G Series Van (G20)

The van will turn over and speed up a little as if it will start and sometimes give a rough, rough idle for 4-5 seconds, but will not start, and sometimes will backfire out of the air intake. 2 weeks ago the van ran terrible so we had a cadallidic converter put on which seemed to solve the problem since the van started and ran great that day. They recommended a tune up soon. 3 days later the van would not start with the symptoms given above. I changed spark plugs, got a new battery, changed fuel filter, changed cap and rotor, added fuel, and get same result. I tested spark plugs with a tester and they spark fine on both sides. I visually looked to see if fuel is coming into carb and it looks fine. I am planning to test fuel pressure with a tester soon. The longer I tried to start it, it seems like the engine is trying to lock up. I hesitate to think it is a problem inside the engine, because there weren't really any symptoms the last time I drove the van that would point at internal damage, but maybe there won't be. I checked the oil and it seems fine. Does anyone have any ideas aside from testing fuel pressure? I may also test the compression. thanks One other small note is that a few days after the van would not start a random person knocks at the door and asks if I am keeping the van. Part of me wonders if they did something to the van? maybe that is just my pessimistic side. thanks again

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2 answers 3 comments
You may check the crank sensor and the cam sensor, from the sound of what you are saying it sounds like it may be firing in the wrong order and if you have done all that yo say you have id check there next there a little bit of money but fairly easy to change.
Just had the exact same problem. I could have written your paragraph. I changed the fule pump. Fixed it! You can check the fule pressure, but what I found is that the pump was pumping just enough to turn over and fill the throttle body with fule, but it wouldn't run longer than 4 or 5 seconds. Changed the fule pump and it runs like a champ. The fule pump is only about $55 at Autozone.
Just to follow up on my original post, who knows what my original problem was as I had basically changed everything one by one that could have possibly been related. Anyhow, what ended up fixing the problem is that after looking at several online diagrams of the distributer cap I realized that I had the correct firing order, however, the wires were shifted over one pin and were therefore firing out of order. I shifted them over one just to try it and it ran like a dream and has every since. Hope this helps someone else.
Just saw this post, am having a very similar issue and my last resort is the fuel pump. I've got a 93 Chevy Van (G20), have never replaced the fuel pump, where exactly is it located? Near the gas tank? What type of tools will I need? Will I need any type of floor jack? Any help will be greatly appreciated. Thank you.
fuel pump is inside the tank. must drop the tank, special tool recommended to remove and replace pump.