Re: Recirculation Breather
The other thing here is that the idle mixture volume set for the engine, in other words the idle mixture, includes a calibrated amount of bleed air drawn into the engine via the PCV valve. This air is initially drawn from the air claener side of the throttle body, passing into the back of the rocker cover via a rubbber hose. It is then drawn by engine suction (via the PCV valve) across the rocker gear chamber, collecting engine fumes to be burnt along the way. The PCV valve regulates the amount of bleed by a combination of a orifice and the tension of the plunger spring. On overrun, it shuts fully to ensure max. vacuum for brake servo assistance.
To see the PCV's effect on idling particularly, remove it from the grommet while the engine is running. The idle may change a bit, especially if your engine is a bit old and fumey. If you then put your thumb over the opening on the valve (that should be sucking) and stop the air bleed, the engine idle will certainly change, as you have dramatically upset the idle air/fuel ratio. So the best advice is to leave it as it is - working.