Chances are it is a stuck fuel injector, or possibly a vacuum line leak.
The 2008 Subaru Impreza has a flat 4 engine.
The 2003 Subaru Impreza has a flat 4 engine.
The 2006 Subaru Impreza has a flat 4 engine.
The 2014 Subaru Impreza has a flat 4 engine.
The 2013 Subaru Impreza has a flat 4 engine.
The 2005 Subaru Impreza has a flat 4 engine.
The 2002 Subaru Impreza has a flat 4 engine.
The 2011 Subaru Impreza has a flat 4 engine.
The 2009 Subaru Impreza has a flat 4 engine.
The 2001 Subaru Impreza has a flat 4 engine.
The 2010 Subaru Impreza has a flat 4 engine.
The 2012 Subaru Impreza has a flat 4 engine.