A bad EGR Valve can cause poor or rough idle, bucking on acceleration, illuminated Check Engine Light, or a combination of these. Then your Mustang EGR Valve may be to blame. An EGR Valve, or Exhaust Gas Recirculation Valve, is a component of the emissions system. It's fucntion is to recycle a small amount of exhaust gas back in to the intake air to help cool the combustion process for a more complete burn of the air/fuel mixture. This also helps prevent the formation of harmful Oxides of Nitrogen, or NOx gasses, making your Mustang's spent exhaust cleaner.