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My Subaru 4 cyd.  runs rough on damp days or when not used of several days. After driving about 10 minutes it begins running normal. Any thoughts.... Thanks  

what year.,???

can you open the hood at night.. in a dark area... start the motor.. see if you can see any arcing from the coil....

i have a friend who keeps a squirt bottle with a little salt in it.. and another with clean water...   he squirts the coil with the lightly salted water.. if the engine dies.. the coil needs replacing.. he then washes the slightly salty water off with clear water...  

if you have a subaru  old enough to have a distributer..  the coil could still be bad..
it could also be a cracked cap.. or moisture in the cap...