Backfires are normal. My 91 GT is an EFI car with headers, an X pipe, and flowmasters, and it backfires on deceleration. Pretty much any car with exhaust mods is going to backfire on deceleration, especially these.. No worries, unless it backfires during acceleration or cruise or the backfires sound like they're going to rip something apart.
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'91 Mustang GT 5.0 AOD Daily Driver
Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI
13.73 at 103.8, ~2.2 60'
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