408 popping/ backfire under load
 

Ok so here's the thing. I have a 408 stroker in my 99 Silverado. Btr stage 2 cam and ls3 heads long tube headers. When I first got the truck going it ran fine. After a couple weeks I changed the exhaust to a magnaflow and went on a 120 mile road trip towing a truck frame. No problems. The next day I went to drive the truck and under heavy acceleration (only under load) the truck sputters and sounds like it backfires through the exhaust. I can rev the engine in park or nuetral and it's fine. It misfires when it does this as well. Where should I start. It's a fresh built engine so I wouldn't think it's mechanical. Plugs? Tune? Wires? Any help or previous experience appreciated.

Check afr and fuel pressure. It sounds like your fuel pump could be going south. If both afr and fuel pressure is good, look at plugs and wires.

I vote fuel pump or burned plug wire.

Plugs/Wires would be the first thing I checked. No spark causing misfire and unburnt fuel in exhaust. Depending on what you find with the plugs, may need to investigate the tune as well. Sounds like you could have an overly rich condition.

It would have to be extremely rich to make it backfire or pop. and what are the odds of it suddenly going rich after it has been running just fine.

a bad exhaust leak will cause over rich fuel mixture

I had something like this happen to my 03 Yukon the o2's were reading crazy, crazy maF codes, sputtering, fine one day and crap the next. Changed plugs, wires, o2's, cleaned maF even swapped maF and still had the rich codes. Said screw it and changed the fuel pressure regulator and it was good as new. I had very good fuel pressure readings and would start up each time just ran like crap. Fuel pressure reg for he win on my part so that might be worth a check?

i made a video of what it does on acceleration. Is this what it was like?
https://youtu.be/zB_53EnkvI4

We,ve hit on everything to check. Get to checking :)

I plan on it, my work is closed where I have all my tools. I was more or less asking if it could be tune related whereas I have no control over it. I do appreciate the suggestions tho. To be honest I was hoping something other than spark plugs would be suggested. With the headers on there it's near impossible to even get a socket on them. I'll need to loosen the headers and move them