2004 LS1 in 69 chevelle. Was running, but had not been tuned yet. took car in for paint and while it was there, they said it wouldn't start anymore. I measured power to fuel pump for 2 seconds during prime and then nothing while cranking. then when I stop cranking, power comes back for another couple of seconds to prime again. I am thinking maybe crank position sensor.
Any ideas?

 

Start with the easy stuff, check all the fuses and grounds first. sitting in a body shop shouldn't kill a crank sensor. How long has it been in there? It's possible that the injectors have stuck if it's been a while due to the crap fuel we have these days.

 

This was the problem with mine, doing the same as originally posted, after a short time of not running while doing the swap...

 

sounds to me, like u are not keeping 12v power while cranking over. Seems to have 12v with initial key on (which works fuel pump) but none while cranking. Check your main igniton relay and wiring.

 

I already pulled and cleaned fuel injectors since that was their first answer.