V6 vortex 4x4
Has major problems starting after it sits overnight , if your lucky and you get it running it will restart perfect....it's kicking me around.NO CODES
I've replaced cam and cranksensor ,tps,fuel pump,ignition module , new distributor,fuel pressure is good , I originally thought the passlock system was the problem but it does same thing with no flashing security, usually if you disconnect the battery, wait a few, reconnect it starts ( no every time ) I can jump the fuel relay force run and it still won't start, exactly what happen is turn key to start engine starts for 2 seconds and stops ,try again does same thing , if your lucky and it starts with no cut off it runs just fine, and will restart fine but let it sit several hours and same issue over and over and Im stumped, please help
Thanks
David
If it's a problem with GM's "Passlock" system you may be able to bypass it.
when it did start it throwed me a code P1351
Probe the ignition control module harness connector terminal B with a DMM connected to a ground.
With the DMM set to the AC scale, crank the engine.
Monitor the voltage reading. It should be between 1-4V
Fuel psi is spot on , new pump and filter and I do have gauges, the old computer never coded at all when it wouldn't start , New computer did outa the gate , after on off attempt crank if finally started , I promptly erased the code , restarted and code didn't show, but after about 3 hours attempted a start and there the code and it didn't start right away, same scenero ...after a few attempts it starts again and will repeat start like nothing ever happened, let it sit , it takes another dump. WTF , I repair half million dollar machines at work daily but jimmy here is a pain , I'm sure the code is correct as per my reading, the start function and timing control tied to it sounds right,I already did the module and it changed nothing , I don't get why original computer didn't code...old firmware I guess , looks like I gotta find a wiring diagram to find it , I hate wiring.Im a wrench tech, we pass wires off to controls 
unfortunately, changing parts doesn't do much to help pinpoint issues. In fact, it often just creates new issues.
Is this the 4.3 engine or 2.2?
4.3 vortec
here you go
Thanks ! I will check it out asap, tomorrow if the weather lets me , I really need this thing fixed and reliable for the wife.
Where are you? Reset the computer by disconnecting the battery so I'm assuming you have a main computer failure or wiring to the main computer that's failing
I'm in north Georgia, I don't have a problem changing out the computer, even considering buying one that's had all the passcode crap removed at the tune of 250, this is my wife's , low milage for it's age, not gonna trash it, I do industrial maintenance , but Mr. Jimmy has been handing it to me, what's your take on ecu's with passcode deleted in program, I sure hope computer fixes issue, wiring is no fun ., thanks Scotty (my wife says your the best )
Thank you guys so much, really the ecm is about all that's left, I'll do a leak test on the grounds at the computer before I rush out and buy one, I always heard GMC was short for "Gota mechanic coming " I will be sure to report back with my findings.
Wire
Update ! So I put a computer in it today and it did the same thing EXCEPT when it did start it throwed me a code P1351 , I put the electronic module on it already......what I've read about that code it sounds like it the culprit, What to do??? Cut into the harness tracing the wires , any and all help I'd sure thank you.