Engine problem.
 

I just got through replacing both head gaskets, lower and upper intake manifolds and all that fun stuff. After installing everything, I tried to crank it. It turned over, and cranked up fast, but seemed like it was missing out a little bit. Now when i try and crank it, it will run well (minus the slight missing out) and within' seconds die.
Any ideas as to what might be to blame?

Any Engine Codes?

Two engine codes, one check gauges (for the low battery) and reduced engine power.
No idead why the second code appeared.

Year? Motor? Most common issue is valve adjustment.

Its a 2002 3800 series 2.
I hope its not valve adjustment, the reason why i had to do all that work on it is because when i was working on my car recently i dropped a bolt into one of the cylinders and it was stopping the valve from working correctly, i just assumed that after taking the bolt out that it would work.

Did you check the spark plug for damage? Otherwise it is possible that either the valves or pistons or both got damaged by the bolt. This will cause bad compression and misfire.

I checked the spark plugs for damage. There was one that needed to be replaced and I replaced it. I'm going to replace all spark plugs soon. I checked the piston and there was a slight scratch in it. The valve itself didn't show any signs of damage. The valve seat and spring didn't have any noticeable damage either.

You said you have two codes, but don't mention what they were. Unless the lights you speak of are what you are saying are the codes. From what I have read, Reduced Engine Power pertains to the electronic throttle not working. Low voltage would be from an alternator that isn't working. When are said lights on?

Im not sure of the codes, but the lights that come up when the engine is starting. Is there anything else that reduced engine power might be?
The lights came on when i was driving up and down a hill, then it died.

Also the engine was doing a little bit of loud backfiring.