blew the og motor in my 96 sierra, pulled an identical running and driving motor and dropped it in. couldn't get it to start and have spark fuel and air. hit a road bump and had to leave it sit for a while. come back to #2 cylinder full of water, flushed it and put new plugs and wires in it. still couldn't get it to start. it cranks and fires but no start. pulled distributor and reset timing. it started but ran like poop, stalling if I am not giving it gas from the pedal. realized camshaft position sensor was unplugged from distributor so plugged it in and it stalls, won't start with it plugged in. any ideas?

 

it probably has a blown head gasket
Before installing the engine, did you do a cylinder compression test?