I'm having a misfire problem after installing a new exhaust manifold. It seems to be fine until the car warms all the way up. Then it will start to miss around 3k rpms. I know it would need to be retuned for the manifold anyways and that's all I thought it was until I put my old manifold back on and I have the same issue. Car ran fine before and that's all I did. Can a leak at the manifold cause this? Any help is appreciated