Chevrolet Light Duty Truck 1973, Руководство по обслуживанию

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6T-2 E M IS S IO N CONTROL SYSTEM S
Fig. 3a—PCV Valve Cross Section
internal dimensions are such to produce the desired
flow requirements.
Fig. 4a—PCV Valve Position • Idle or Low Speed
air
During periods of deceleration and idle, manifold
vacuum is high. The high vacuum overcomes the force of
the valve spring, and the valve bottoms in the manifold
end of the valve housing. This does not completely stop
the flow but it does restrict the flow of crankcase vapors
to the intake manifold (Fig. 4a).
When the engine is accelerated or operated at constant
speed, intake manifold vacuum is less than at idle or
during deceleration. The spring force is stronger than the
vacuum pull during this mode so the valve is forced
toward the crankcase end of the valve housing. With the
valve in this position, more crankcase vapors flow into
the intake manifold (Fig. 5a).
In the event of a backfire, the valve plunger is forced
back and seated against the inlet of the valve body. This
prevents the backfire from traveling through the valve
and connecting hose into the crankcase (Fig.
6
a). If the
backfire was allowed to enter the crankcase, it would
ignite the volatile crankcase blow-by gases and result in
a sizable explosion.
It should be noted that additional air is permitted to
enter the intake manifold when positive crankcase
ventilation is used. However, the carburetor used with
this system is calibrated to compensate for the air plus
blow-by gas that enters the intake manifold from the
crankcase.
As can be seen from the foregoing discussion, the
Positive Crankcase Ventilation system provides for
proper air flow through it during all operating
conditions. In addition, the crankcase vapors are
reburned rather than exhausted to the atmosphere - thus
helping reduce our air pollution problem.
So far, we have discussed the two major benefits of the
PCV system - namely, venting of detrimental crankcase
vapors and air pollution reduction. There is a third
benefit of this system that is equally as important to an
owner; that is - Better Gas Economy. The recirculated
vapor returned to the intake manifold is a combustible
mixture. It, in effect, becomes fuel for operation when
mixed with the air fuel ratio from the carburetor.
In summary then, the PCV system provides:
Fig. 5a—PCV Valve Position High Speed or Cruise
Operation
Fig. 6a—PCV Valve Position • Engine “ OFF” or
“ Backfire" Condition
• Longer Engine Life - Draws harmful blow-by gases
out of the crankcase.
• Reduction of Air Pollution - Reburns crankcase
HIGH SPEED
LOW
MANIFOLD
VACUUM
MAXIMUM
<0
FLOW
PLUNGER IN OPEN POSITION
LIGHT DUTY TRUCK SERVICE MANUAL