Quote:
Originally Posted by scutyde
The hole is for the optional secondary pulsed air system . A metal pipe is attached to it an blows fresh air into the the head. The lower half is different in this setup so that that the air reaches the exhaust gases and causes an "afterburn". The point is to heat up the catalytic converter faster so to improve the overall emissions.
Normally the hole is empty on cars with no PAIR system and no oil should come out of it.
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This is spot on. There is also zero reason to plug it. If you have oil beneath your spark plug cover you either have (or had, depending on its colour) a bad PCV system, or your oil filler cap gasket needs replacing. In my case it was the latter.