im pretty sure my its the original engine inside my fd but im not sure because i havent really pulled it out or anything. well i installed a hi flow cat on it not too long ago and its so loud compared to stock and it feels as though the car shakes sometimes. well when i start my car up it feels alright but after about 20 seconds it sounds like it starts to hesitate and not make a consistent idle noise. when it hesitates the whole car seems to shake but then it continues to idle normally. when i give it some throttle it revs normally but then it comes down from the revs it goes really slow and seems to heisitate. any ideas what its?

 

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