Back in the good old days, the crankcase gases just vented to atmosphere. To help stop polluting, a hose was attached to the top of the block and routed to the carburator which had natural vacuum. The PCV valve was added to restrict the amount of air going through the hose so the RPMs didn't go through the roof (unmetered air). Unfortunately, the PCV valve gets crudded up and blocks the flow to the carb/fuel injection manifold.

Because the crankcase is sealed, as is most of the cylinder head, unmetered air is at a minimun from the PCV and will not change idle speed.

Klaus
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There is no present time, just the past and future