I removed all diesel components and wiring from my 95 c3500 and replaced it all with 98 k1500 gas motor, wiring and all components. It fires and runs for 2 seconds or so and then dies.
I have checked the VATS passlock and there is no "security light" issue - it goes off after 3 or 4 seconds. The fuel pressure is around 56 PSI without the engine running. The system has a new filter (inline) and the unit was running in the old 98 body. I don't think the fuel system is the problem, but I don't know how to check and see.
I'm thinking it might be some sort of a computer thing and I don't know the procedures for checking - or repairing.
You help is appreciated
It can get complicated with a swap like that. But the fact it starts and dies is typical to the anti-theft system.
Whenever you replace parts on this system, a reprogramming is required to make it work.
For example: If you're using the original Passlock sensor in the column, and the original Passlock module, the codes will not match the PCM -- and requires reprogramming at a Chevy service department.
The thing that worries me is how far you went with the wiring harness replacement. Did it include all the Passlock components and wiring? If not, you may have a mismatch of harnesses, also.