Neeed Helpp..

[MyfirstCar]

So when i was first buying the car the enngine sounded bad cuz when i turned it on it made lik a pop every 5 seconds and when it was being drivin it would occaisilly nudge u while driving (lik u keep gasing and breaking) , and the reason why it does that supposely is cuz the catalyic converter got messed up cuz it almost got jacked by someone. so do u think it might be the engine thats bad or the CC that makes sound that it makes and does what it does?? also i got it looked at and it said my compression is low do u think the catalyic converter has a part in that too??