ok so my truck now runs, kind of
what happens is most of the time it will start up on the first try, tho every now and then it will take it a couple seconds to fire up, then it will run and you can hear the exhaust popping every now and then like a backfire. when you go to take off, especially in reverse, it will take off and stall out. when it stalls out the CEL comes on. last time i checked i had codes 21 and 51. also when i went off roading the other day, i noticed that it stalls out when you go to put it under load, it will sit and idle all day long but as soon as you ask power it dies. any help GREATLY appreciated.
BTW truck has new fuel pump, screen, lines, dizzy cap, rotor button, plugs, plug wires, air filter, efi main relay, checked the circut opening relay and its good, fuel pump is on a switch.

 

fuel pump works fine, it is a new unit, and i tested it anyway
CEL comes on whenever it hesitates/dies but goes right off
i have gotten codes 21(main o2) and 51(ac switch of something like that) out of it becore, planned on replaacing/re-wireing o2
can someone link me to a vac diagram?

 

You have never had it running normaly, so I would start with the basics. Tune up, compression test( you said you had a mechanic check it , but you are not sure), valve timing ,valve adjustment , fuel pressure( use a gauge, dont guess) , remove and clean injectors. If you want to work on it yourself , you have to buy basic tools or you are wasting your time.