Sometimes the intake pipes on Lycoming engines can come loose. This will cause an intake leak, usually indicated by a rough idle and higher EGT on the affected cylinder. The leak will also leave a tell-tale sign of dark material around the pipe from fretting. It takes a special internal swaging tool to reseat the pipe.
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I had loose intake pipes starting around 1000 hrs. Tried some proseal around there which worked as a stopgap but eventually I found an engine shop that was willing to loan me the swaging tool. Lucky! Now all tight and good.