Car only backfires when accelerating at any rpm

Waterchicken said:

thank you for the knowledge, would you suggest I mess with the wiring on the o2 sensor or put on a stock header? Or something else? Again thank you

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Do you have any Forced Injection? Or other power adder mods or are you just doing these headers for the sake of sound? If you don't have any power adders then you are just wasting money with a Megan racing header and other stuff because the only way that you are going to get the vehicle to run right is by tuning it which could mean putting an aftermarket ECU into a N/A car which would be a stupid way to waste 500-2000 dollars. Instead, keep everything stock until the second cat and after that, you can put all the fun sound mods on the car.