back firing would be a timing issue... is it backfiring through the exhaust, or the intake? I'm thinking, if it's not the PCM, you're timing is being retarded to much. The turbo spools up really fast at partial throttle around 2500 rpm - the intake mods would make it even more so, and the PCM might be retarding the ignition to accomodate a potential lean condition. I'm not sure why this would only effect some cars though.. Have you tried putting the factory airbox back on and seeing if it still has problems?
I'd brainstorm some, try to figure out what scenerios would trigger the pcm to retard the timing.. I'm at work, but I'll think it out some when I get home tonight.
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2003 Electric Blue GT
- Greddy E0-1, boost and AFR guages
- Custom exhaust: no muffler, side exit.
- Replaced air box with open end cone filter.
- Stage 1
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