In my case it was a bad spark plug well gasket. Oil gets inside the tube and causes cylinder misfire. You always need to replace the valve cover gasket as well. On 2002-2006 model spark plug seals are embedded into the valve cover and it’s impossible to replace just the seals. You’ll have to buy valve cover. You can find step by step instructions and more details such as torque specs and sequence, silicone gasket maker application, tools and parts needed, etc. in the following video: http://www.youtube.com/watch?v=JQmMdfEGw...

Check out these videos on Youtube: Back Housing Replacement iPod Touch / Full Disassembly / Taking Apart - 4th Generation http://youtu.be/0ScpTGGqb3U Back Housing Replacement - iPod Touch 4th Generation - Part 2 http://youtu.be/SBRfagwk4II It might be helpful to see how it's done step by step.