I have a 2002 focus SPI with a very erratic/rough idle. It started out as a slight rough idle and has progressively gotten worse to the point where it doesn't want to idle at all. It drives great with no noticeable performance issues it just won't idle. I looked the engine over and over for vacuum leaks but couldn't find any. Checked the EGR valve. Checked the IAC valve. I replaced the plugs and wires. I also replaced the upstream o2 sensor which didn't fix the problem. The thing is when I was replacing the o2 sensor I noticed the exhaust manifold has a decent size crack in it. Would this cause a rough idle issue? I'm not even sure if the crack goes all of the way through the manifold. Car keeps throwing code P0171 bank one system too lean. Would this crack in the manifold cause a lean running condition? Any thoughts or input is much appreciated.