PCV Breather - how does it work?

[AndyTTR]

Here's an example pic of the PCV breather system on most Toyotas i've seen.


(seen here on a '93 2JZGE).

There's usually two breather points, one before the throttle, and one after which always has one-way valve (circled in green) - to stop the engine sucking air past the throttle i assume.

What I am trying to understand is why they did it this way. Why are there two breathers? Why is one before the throttle, and one after? Does the system rely on vacuum from the engine side of the throttle to work? If i were to put a catch can in this system, where would be the best place to connect it?

On my car, the catch can is only connected between points 2 and 4 in this pic, am I only catching half the blow by? The can does catch oil, but I've not noticed any oil residue on the throttle butterfly since i cleaned it 6 months ago.

Is the best way to set up the catch can connecting points 1 and 2 together, run these to the catch can and vent it to point 4 with a one way valve?

I've has a few bourbons and am in a contemplative mood... i need to understand this!

[AndyTTR]

i just re-searched and found this thread;

http://www.toymods.net/forums/showth...ght=catch+blow

So... the pre-throttle pipe allows filtered air back into the engine to balance the vacuum created when the one-way valve is open. But does this 'fresh air' pipe connect directly to the crankcase or does it vent into the same void as the post-throttle breather?

I think im getting wrapped around the axles on this point - when does the PCV system actually work? At wide open throttle, closed throttle and higher rpm (eg. engine braking = vacuum), or at idle? Might have to go down and pull that valve out tomorrow to find out when it opens and closes.

[thechuckster]

the PCV system works thus:
-when at high manifold vacuum, the valve in the rocker-cover allows crankcase fumes to enter the manifold directly, the crankcase (at the same time) draws in air from the port ahead of the throttle
-when at low manifold vacuum, excess fumes from the crankcase are allowed to escape thru the port ahead of the throttle where they are then sucked into motor and burnt. The valve in the rocker cover is closed at this point and not doing anything.

The PCV (Positive Crankcase Ventilation) system comprised of two elements - not just the valve - so it works all the time.


The correct way to plumb the catchcan would be:
1 - to the can
2 - remove the valve then port to the can
3 - top outlet from the can to here
and if you want to be strictly legal, 4 to the can with the PCV-valve inline so the engine only scavanges fumes from the can at high manifold vacuum

however, many people skip 3 & 4 having the can just vent directly to the atmosphere.

[MWP]

Well made PCV systems (do some research, there are aftermarket systems) always keep the crank case under a vacuum.
This stops the chance of oil leaks and also can improve engine performance as the piston rings seal better and reduce blow-by.

I have read some NASCARs actually use a dedicated belt driven pump to keep the crankcase under vacuum for increased performance.

Factory setups are normally just there to remove dirty air from the crankcase.
Obviously dirty blowby air in the crankcase fouls the engine oil quicker.