Chevrolet Light Duty Truck 1973, Service Manual

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E M I S S I O N C O N T R O L S Y S T E M S 6 T -3
vapors rather than exhausting them into the blow-by gases by returning them to the intake
atmosphere. manifold.
• Increases Gasoline Economy - Reclaims unburned
DIAGNOSIS
Engine operating conditions indicative of improper
Positive Crankcase Ventilation System operation usually
are accompanied by one or more of the following
conditions:
1. Rough Idle
2. Oil Present in Air Cleaner
3. Oil Leaks
4. Excessive Oil Sludging or Diluting
If any of the above conditions are observed, corrective
action; such as: PCV valve replacement, hose replace­
ment flame arrestor cleaning or replacement or PCV
breather (located in the air cleaner) replacement, should
be accomplished as required to eliminate the
malfunction.
Proper operation of the PCV system is dependent upon a
sealed engine. If oil sludging or dilution is noted, and the
PCV system is functioning properly, check engine for
possible cause and correct to ensure that system will
function as intended.
CONTROLLED COMBUSTION SYSTEM
THEORY OF OPERATION
In essence the Controlled Combustion System (CCS)
increases combustion efficiency through carburetor and
distributor calibrations and by increasing engine
operating temperature (Fig. lb).
Although the CCS system imposes no additional
components, other than a special air cleaner assembly
that is used in conjunction with it, an understanding of
how exhaust emissions are reduced would be beneficial.
In general it can be said that CCS carburetors are
calibrated leaner and timing is retarded during low
speed or deceleration operations as compared to systems
used in prior years. However, this does not always hold
true. The system was individually developed on each
engine-transmission combination. Various adjustments
were made to reduce the hydrocarbon and carbon
monoxide emissions. The exact provision or adjustment
made was not the same for each engine-transmission
combination.
The following provisions have been made on all CCS
systems. Exceptions to these providions will be noted.
Carburetors
Most carburetors used with CCS system are the same as
carburetors used in prior years but specially calibrated
throughout their driving ranges for optimum emission
control as well as the usual parameters of vehicle
performance, economy and driving feel.
All CCS carburetors have an internal provision that
allows production to refine part throttle calibration to
meet very accurate air/fuel ratios and thus reduce
emissions in this sensitive area.
Another feature incorporated in the CCS carburetors is
an “Idle Fuel Limiting” feature. This is nothing more
than a fixed orifice at the base of the idle mixture screw.
This insures that even if the idle mixture screw is turned
out too far, the fuel enrichment will not greatly affect
exhaust emission reduction.
NOTE: All carburetors are equipped with
idle mixture limiter caps; no attempt should
be made to adjust mixture - do not remove
mixture screw caps.
Distributors and Timing
As mentioned initial ignition timing is slightly retarded
over previous years. If you took two equal engines of
which one had its timing retarded with respect to the
other, a wider throttle plate opening would be required
to obtain the same RPM on the engine with retarded
spark. With a wider throttle plate opening, more air can
Fig. lb —Schematic of Controlled Combustion System
LIGHT DUTY TRUCK SERVICE MANUAL
NEW CAR BU R ETO R
LE A N M IXTU RES .
EN G IN E IDLE
INC R EA SED
HIGH TEM P ER A T U R E
TH ER M O S TA T FOR
E N G IN E CO O LAN T
K EY TO C .C .S. S YS TEM
HEdTEO UNDERHOOD M l
UH0ERH00D MR
C6®E! HOT EXHAUST
C O N T R O LLED
IN TA K E AIR
H EA T E XC H A N G ED
FROM E X H A U S T
M A N IFO LD TO
U NDER HO OD
AIR FLO W