I just did a 5.3 swap into my 85 c10 now im getting a loud backfire sometimes when I start the truck .it starts up and runs but seems to have a slight miss it didn't do it when it was in the donor truck . im thinking a injector is dumping too much fuel. or I might have the harness that goes from the big plug to the coils could be backwards . or maybe a bad spark plug. I did start the truck a few times and let it run for about 5 minutes without the o2 sensors or any exhaust . I had the drivers side backwards when I first installed the engine and it wouldn't start it kept popping like it was out of time I switch the drivers side coil harness and it started . but now im having this problem .not sure if I need to swap the passenger side around if you guys and tell me what color wires should be on the front coil packs it might help me eliminate one possibility . the harness is from a 2002 chevy suburban 5.3
thanks in advance

 

go over to LT1Swap.com he has wiring info. I would start by printing list off and double check your wiring. check grounds as well.

 

ok I figured it out. I just flip flopped the coil wire harness on the passenger side of the motor and the truck woke right up

 

Mine starts but it pops kinda loud how did you flip flop the coils orb what you do thanks !

 

I was reading all your replies about misfire on the 5.3 L. I also put a set of LT headers on and had misfire backfire in cylinder seven. Change plug wires and plugs. Change plug wires first that didn’t fix the problem so then I replaced the plugs now it runs like a champ so I must’ve broke a spark plug.