[dflarsen77]

Black smoke is oil, white smoke is coolant, but from what you describe, not being too sever, it shouldn't cause it to shut off. The back pressure from that Kia muffler should be pretty high, and that could make it stall. I wonder if enough back pressure could cause ring and valve seal problems. If you can, unbolt the exhaust from the manifold or header, let it run and see how it behaves. I can't imagine you'd want to keep that muffler for long even if it isn't causing problems.