I've got a 92 Z28 with a 5.3 out of a 04 Silverado and I have an issue that has been driving me nuts.
Issue: Startup vehicle and it will start and run well for a couple of seconds and then shut off, try and restart it and it cranks but won't start. Checked for spark and no spark is present. Read a few threads on other forums which all pointed towards a bad crank position sensor. Replaced sensor and same issue persist. Hooked up my scanner and indeed it is picking up an rpm signal which rules out the crank sensor or its wiring. The vehicle has a brand new Walbro 255 which kicks on when the ignition is turned to on. I'm running out of ideas as to what may be the cause of this issue. Any help would be greatly appreciated.

 

You didn't make mention of pulling diagnostic codes.

 

Did you remove vats?