I assume you are talking about the pcv (positive crankcase ventilation) and not having the hose connected to the intake. The intake draws vacuum from the crankcase to pull gases out. The pcv valve opens and closes. I think it is like a check valve just making sure things go in only one direction. Once the gases are pulled into the intake they are reintroduced to the cylinders to burn more of the gases. If you disconnect it you should have a filter or breather in place where the pcv valve sits so that it can vent to atmosphere like older engines. If you just keep the valve there it may not open without vacuum. If the crankcase can't vent you will have issues with churning oil, gases putting slight pressure on gaskets and bearing also valve guides causing leaks.
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