Okay, so this is a 95 s-10 with a 98 rebuilt motor. I'm trying to help my neighbor so i dont know exact details on what all was done. problem started couple weeks after the rebuilt engine was dropped, someone put low grade gas and had been putting medium grade. now it cranks but it put put puts and sputters and has aggresive backfiring. it'll idle alright for a little bit. thinking air/fuel ratio is getting screwed up somewhere. i thought (i do honda etc..) maybe the ecu for that was in the truck needed one to match. ie: 95 body/ecu with 98 motor... needs 98 ecu to match with motor? but i guess thats not a big deal with these. its 2.2 manual. i will put up pics, but frankly dont know what to do. have changed vacuum hoses etc.. theres alot of wires etc they never got plugged back up, but shouldnt be anything of that much importance to the problem?

 

I would start with some octane boost, then try changing the fuel filter, then plugs & wires, then rip your hair out. You say it went for a few weeks, so the swap motor ran good for a bit before the low grade gas, so it seems the swap took OK. Maybe water in the gas did bad things.