But why does the maf sensor need to know about air that is coming from the crank case, as the tube that goes into the air tube draws the air from the crankcase when the throttle plate is open and its all after the maf sensor to begin with. Im trying to figure out both of your guys reasoning. I also have a breather on my valve cover just like quick and have never had any problems and neither have people that i know that have the same set up with out a tune. Not saying that your wrong or not.
Russ got it.
The PCV system uses vacuum from the plenum to draw clean air through the crankcase to vent it. The air is supplied from the intake tube, before the throttle body of course so it's not restricted by throttle position. Air is not drawn FROM the crankcase through that tube into the intake tube (pre-TB) like you're saying - that would not only render the PCV system useless at lower throttle position but since the PCV valve is on the other side would allow the entire intake to be coated in crankcase oil vapor
In addition, the PCV is designed to only flow air the correct way - that's why it has the small weight inside. Air can flow through it to the top and into the plenum, but not down and back through the crankcase.
All airflow into the engine is metered by the MAF to aide control of the air-fuel trim. By having the PCV system draw air from a breather rather than the intake tube, you are drawing un-metered air through the crankcase and into the plenum. This would cause the car to behave just like a vacuum leak - there is extra air being sucked in that the MAF did not see. And just like a vacuum leak, this extra un-metered air causes the air/fuel ratio to go slightly lean as the MAF is not metering this air, only the air passing through, so the PCM is using a air/fuel map for less airflow than you actually have and causes the HEGOs to see a leaner condition which has to be corrected.
Again, you also have a dynotune where this can be corrected. But someone with a stock vehicle, or one without a tune with this in mind WILL run leaner because of it. Probably not enough to cause damage or anything, but leaner none-the-less.
What I'm saying is on a stock vehicle it may work
, but it's not right