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Hello, my name is ***** ***** I am a professional here at Just Answer. I have noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.If its now backfiring and it was not before then I would suggest there is something defective with the programming they did and that should be checked. If in fact the engine is backfiring as you are saying that is not considered normal at all. I would have them recheck there programming .