In the above diagram, you can see the high vacuum (blue) caused by the downward "sucking" of the pistons. Since the throttle valve is closed, the vacuum is strong as it is like sucking on a bottle of Coke. The "chamber" is fixed and essentially closed. All vacuum lines connected to the manifold (such as brake booster, distributor Vac Advance, and pollution control lines) also have the same vacuum but they are not shown to avoid complexity. btw the typical vacuum for a healthy Z is 17" to 21" of mercury (Hg) at idle
Of course, we know there is a small air leak across the throttle body via the "idle set screw". This allows some oxygen to enter the combustion chamber and the car to idle! This small air flow causes a slight venturi effect vacuum in the chamber between the air filter and the TB valve (pink) but it is a weak vacuum.
Please note that the faster the engine turns, the less manifold vacuum exists.
Finally, the exploding gases of combustion leak past the rings and valve seals. These high temperature and high pressure gases cause the internal engine block and valve cover to experience positive pressure (green). The faster the engine turns, the more blow-by occurs. Unfortunately, this so-called "blow by" vapor is filled with nasty chemical compounds such as explosive hydrocarbons, moisture and aggressive acids. These MUST be removed from the engine at all times!
The Positive Crank Case Valve ( PCV ) and the Blow-By Upper Hose, work as a team to recycle the noxious blow by gases back into the intake manifold where they will be burned. This acts as a pollution control.
The PCV is just a spring loaded valve. The spring prevents the high vacuum at idle from sucking too much crank case air. (As mentioned above, blow by is proportionate to engine revs so there is minimal at idle. As the engine revs faster and the blow-by pressure builds and the vacuum in the manifold decreases, the PCV valve opens to allow more blow-by to flow into the manifold. The PCV valve also has a reverse flow stop so an explosion in the intake will not flow into the hydrocarbon rich crank case and cause the engine to explode! Likewise, the flame arrestor at the top blow-by upper hose serves the same purpose.
The Blow-By Upper Hose does not have direct manifold vaccum thus it wisps away crank case vapors rather than aggressively sucking the vapor by applying direct manifold vacuum (note: in early FI Z cars, the upper hose was directly connected to the high vacuum of the manifold via a leaky Aux Air Regulator, this caused too much suction and tons of junk would be pulled into the manifold as well, the old routing allowed the high vacuum (blue) to extend into the valve cover area. This resulted in a huge vacuum leak when you removed the oil filler cap.
Note: when the rpms are low and the manifold is sucking blow-by via the PVC, excess suction may cause fresh air to be sucked into the valve cover via the Blow-by Upper Hose. When the rpms are high and the Blow-By is great, the upper blow-by hose actually has more vacuum than the PCV valve thus it sucks the blow by out of the crank case and the PCV is closed.
The bottom line for a FI L28E is: blow by gets sucked out and fresh air is sucked in. At most RPM's, the PCV sucks the crank case blow-by and the Upper Hose replenishes the crank case with fresh air. At high RPM's. the cycle can reverse where-by the Upper Hose sucks the crankcase blow-by .
If the PCV valve or PCV hose is clogged or malfunctioning, lots of acidic, watery, carbon-y deposits will accumulate in the engine and cause corrosion and wear. As well, the increased crank case pressure will start blowing oil out of seals. Replace the PCV every year and clean and inspect the PCV hose. It often clogs or simply fails because it is so close to the hot manifold.
Here is a great overview of how the PCV valve works pcv.pdf
More great info on the PCV Valve
From 280z FSM:
Crankcase Emission Control System Description
- The system returns blow-by gas to both the intake manifold and throttle chamber.
- The positive crankcase ventilation (PCV) valve is provided to conduct crankcase blow-by gas to the intake manifold.
- During partial throttle operation of the engine, the intake manifold sucks the blow by gas through the PCV valve.
- Normally, the capacity of the valve is sufficient to handle any blow-by and a small amount of ventilating air.
- The ventilating air is then drawn from the throttle chamber, through the tube connecting throttle chamber to rocker cover, into the crankcase.
- Under full throttle condition, the manifold vacuum is insufficient to draw the blow by flow through the valve, and its flow goes through the tube connection in the reverse direction.
- On cars with an excessively high blow-by,
some of the flow will go through the tube connection to the throttle
chamber under all conditions."