I have a 98 Yukon 5.7 motor that wants to start but doesn't.  I put in a new motor with GM timing cover and crank sensor.  I also put in a new ac delco fuel regulator for spider injectors.    It was running but I did not drive it because I wanted to make sure there was not leaks or problems.   I turned it off one day and when I started it then it instantly turns itself off.  It acts like it wants to start and then it will not.   Coil and spark plugs have fire.  It has 60 psi coming from fuel pump.   I checked wiring harness on fuel injection.  8 pins when you turn on key have power.  All of the 8 other pins as I roll over motor show no pulse.  I was told that it probably the computer.  I put in a rebuilt computer (set with vin of vehicle).   Still same issue.  I have also been told that this vehicle has a security system that might be turning it off because it thinks it is being stolen.  However on the console there is not security light or emblem but there is a security fuse.  I have looked under dash for the security module and can not find one.   Can you think of anything else to try?