Thoughts on backfire?

[XfireZ51]

Originally Posted by KILLSHOTS

The backfiring is definitely happening at the mufflers. Marc mentioned the injectors and asked about the O2 sensors, as well. But the more I think about this, the more it seems like it must be exhaust leaks. 3 weeks ago, the car had straight pipes (except for cats) and was EXTREMELY loud. Also, the DFCO had not been disabled at that point, yet there was virtually no backfire. The injectors were changed in February, so they're not the problem. Bill drove the car thoroughly several times after install and probably would have noted loud backfiring if it was happening then, which tells me the situation is worsening over time. I'm thinking exhaust clamps are loosening with heat cycling. What do you think?

Chris,

That's an important observation. Leak is the first thing I would suspect and inspect for. Decel would cause fresh air to be drawn into hot exhaust. Would also explain lean mixture. Which side is it or is it both? Yes I've had clamps stretch.
First time I ran into this was after putting MagnaFlow muffs on my 84 Xfire. Doing some tuning, and I was constantly needing to add fuel in the "decel region" of the VE table. That's around 20-30kPa. Until one day I walked up behind the car and noticed the left muffler hanging low. Fixed that, and then I needed to take a ton of fuel out. So the O2 sensor can be affected even by a leak quite a bit downstream of it basically making the situation worse.