So my 2003 GTS has been having a really bad misfire on bank one. Like, all three cylinders on bank one are misfiring and running really rich. It's fine until the engine warms up but it's been getting progressively worse. I'm thinking it has something to do with the butterfly valve leaking, but wouldn't that cause it to misfire on all cylinders? I can't really find much information on it. It seems like an issue specific to a mistake I made somewhere along the lines. I'm just wondering what you guy's opinion on the matter would be. If it is because of the plenum does anyone know a good place to purchase a new plenum or a way to repair the butterfly valve? It still opens but the o ring in it basically because brittle plastic and fell apart when I tried looking at it. I know that's part of the issue but could anything else cause it?