i recently bought a 2006 trailblazer when I bought it there wasn't a battery in it. so I put a new battery in and it started and then died.. I started it again and it idled fine. drove it to gas station to put gas in and went to start it and it would run for a few seconds and die. did that for a good 45 min til I gave up. I let it sit for a week and then went back out and cleaned the throttle body and put a fresh battery in it and it started right up and idled great. as soon as I put it in drive or reverse or neutral it dies within 2 seconds. I also noticed when I try to start it in neutral it will crank but will not start but as soon as I put it back in park it starts right back up and idles fine.. where do I began?

 

Welcome from Florida. I would do a fuel pressure check and it probably needs a tune up.