Originally Posted by 911medic
To understand how a breather could allow unmetered air into the intake, you have to first think about the complete PCV airflow path.
The entire system is based off of intake manifold vacuum.
Air is drawn into the crankcase on the "clean side." With the stock configuration, it is drawn via a hose that connects to the air intake tube AFTER the MAF sensor--which is important, because that means the system can account for that volume of air. With the breather cap, the stock hose is eliminated, and instead air is drawn into the clean side through the filtered breather. This volume of air does not pass the MAF sensor, and is therefore "unmetered" by the engine control systems.
The air travels through the crankcase and is eventually drawn out the "dirty side," along with the undesirable stuff that the catch can is meant to prevent being drawn into your intake. While in the crankcase, the fact that it's unmetered makes no difference, as you suggest. It's not becoming a part of combustion here.
This is where the unmetered part comes into play: after the catch can, the air is drawn into the intake manifold and is now part of the airflow that feeds the cylinders for combustion. The engine has calculated (via the MAF sensor) the volume of airflow that is on its way to combustion. Now there is extra air being sucked into the mix.
Now some breathers (like the RX brand) attempt to compensate for this by integrating a one-way checkvalve into the breather. This checkvalve supposedly restricts the amount of airflow into the system to a level that the engine computer can adjust for. Because (as I understand it), in addition to the MAF sensor information, the engine computer also looks at the O2 sensor data of the exhaust gases to determine if the engine is running too rich or too lean, and makes adjustments to compensate.
However, there are limits to the range of adjustments the engine can make, given the information it gets from the MAF sensor. So, if the checkvalve fails, or isn't really "calibrated" from the start to limit the air to within the range, you end up with messed up A/F ratios.
Conversely, if the checkvalve is too restrictive, under heavy vacuum you could potentially damage seals as the intake manifold can create very strong crankcase vacuum.
One other thing, pertinent to breathers with one-way checkvalves that only allow air to be drawn INTO the clean side: Under certain conditions (like WOT, or forced induction), intake manifold vacuum disappears. Now, if you have a buildup of crankcase pressure, it's not being drawn out of the dirty side, and it can't get out of the clean side since there the breather valve only lets air IN, not OUT. There's potential to damage seals that way, too.
Make sense?
If you look at your PCV fitting, there's really no valve. It's just a hollow metal housing with 2 holes on one side and one on the other. The PCV system is a constant flow-through design that is always drawing air through the system as long as there is intake manifold vacuum. The oil pressure is generated after the oil pump (in the upper engine). The crankcase itself is not pressurized by design.
The stock filler tube cap does not vent at all. The stock clean side hose takes care of the PCV makeup air (as described above) and also allows pressure to vent out, if necessary, back into the main air intake tube, ahead of the throttle body. It's a closed system, and has to be, to meet emissions regulations.
If you mean your breather, it depends on which one you have. If it is the RX brand, it doesn't vent out, it only allows air into the engine, which can lead to the issues described above.
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