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slikxb
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220 Posts
Discussion Starter #1
got a question that fries my mind whenever i think about it. pcv's vent + crankcase pressure into the inlet manifold, but crankcase's must be vented anyway to avoid -ive crankcase pressure. therefore my question is why isn't this just causing a major vacuum leak.
 

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Birth, Drift, Death...
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3,039 Posts
the crankcase is vented in to the intake manifold behind the throttle body via the tube at the front of the rocker cover, which has a restriction in it to avoid massive airflow and poor running.

it is also vented just in front of the throttle body (no vacuume there) via the tube at the back of the rocker cover.

the crankcase never has negative pressure in it due to the restriction in the PCV valve (thats why its called a Positive Crankcase Ventilation valve)

the PCV valve is there to suck out all the blowby gasses that leak past the rings when the car is running, and also all the unburned fuel and water etc that condenses into the oil when the car is cold gets sucked out once the motor heats up and it all evaporates into the crankcase.
 

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slikxb
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220 Posts
Discussion Starter #3
What is to stop the intake vacuum from opening the valve rather than the +ive crankcase pressure.
I've sucked on a rocker cover tube and can operate pcv freely.
The tube vented in front of the throttle body is open to atmosperic. I still don't get it. Why doesn't the vacuum hold pcv open and suck through the atmospheric tube?
 

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slikxb
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220 Posts
Discussion Starter #4
I thought any vacuum leak big or small (restricted) would make an engine run like a dog.
 

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Birth, Drift, Death...
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on a MAP based system (like the I6 has) a vacuume leak will not affect performance to such a huge degree - but on the MAF sensored ones it does affect it - not usre what they do there.

the air does get sucked in the atmospheric port, and drawn through the motor, and it mixes with the other gasses in there, then it gets sucked out the PCV valve.

im not quite sure how the actual PCV works - it might be set up so that it only restricts when there is a high vacuume on the end of the pipe - engine vacuume at idle is a lot more than what you can do with your mouth.
 
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