2004 Chevy Impala The body is in mint conditon nice and clean for its year

2004 Chevy Imapla Runs and drives lots of New parts Clean tiltle in hand only problem is it stared smoking alil from the tail pipe my Mechanic said one of the gaskets or seals went bad so i replaced the Motor with a new motor with 100. 00mi and once i started it. it also started smoking alil from the tailpipe my Mechanic says its the Catalytic Converter or the Muffler it self but im done spending money on it i already bought me a new car. But take note the car runs and drives great has no issues everything works. Feel free to message me for more info