Chevy & GMC Vans

Parked the van in the garage when I went away for 3 weeks. Returned and the van won't start. It turns over and initially fires once. The security light comes on. Have tried to start it 12 times, same result. Had problems with this same issue before. The ignition switch was changed and new keys. Problem fixed. 6 months later the same problem. It was the security system. Replaced security system with "New Rockies" brand bypass system. Started. 6 months later I have this problem. Have replaced spark plugs, wires, cap, rotor, fuel and air filter. Any thoughts guys? Anyone had a similar problem after leaving a van for a while?

Seems to me you need to determine if it is a spark or fuel issue. Once that is narrowed, the problem will likely be easy to determine. (Inline spark tester for spark or fuel pressure gauge for fuel flow.) Are you hearing the fuel pump run/prime when you turn the key? Fuel pump is my guess.

Are you getting a nice blue spark from the coil?