jd:

The OE configuration provides air at atmospheric pressure to the inlet of the crankcase ventilation system. At low power settings/throttle openings air flows through that hose into the interior of the engine, and then is extracted through the vent valve/orifice that is located immediately behind the inlet hose, and into the inlet manifold. At high throttle openings/high RPM the flow is partially reversed due to higher crankcase pressure, and the blow-by gasses escape through the inlet hose, and are drawn into the air entering the inlet tract.

If the vent hose is connected directly to the plenum downstream of the throttle, the connection that was originally an inlet to the vent system will become the sole effective exit from the crankcase, and more significantly, will introduce a large amount of un-throttled air/blowby into the plenum, thereby substantially increasing the idle speed, and there will cease to be any source of fresh air into the crankcase.