GMC 1982 Light Duty Truck, Руководство по обслуживанию

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6E-2 ENGINE EMISSION CONTROLS
A IR IN T A K E
OPEN P O S IT IO N
Fig. 6E-3--Deceleration Valve
Diesel Engine
Refer to On-Vehicle Service for Exhaust Gas
Recirculation System general description and service
information on a diesel engine.
EARLY FUEL E V A P O R A T IO N (EFE) SYSTEM
G asoline Engine
The EFE system is used to provide a source of rapid
heat to the engine induction system during cold driveway.
Rapid heating is desirable because it provides for quick fuel
evaporation and more uniform fuel distribution to aid cold
driveability. It also reduces the length of time carburetor
choking is required making reductions in exhaust emission
levels possible.
EFE systems may use a valve which increases the
exhaust gas flow under the intake manifold during cold
engine operation. The valve is vacuum operated and is
controlled by a thermal vacuum switch (TVS) which applies
vacuum when the coolant temperature is below the
calibration value.
Diagnosis and service procedures of the EFE system
can be found in this section.
AIR M A N A G E M E N T SYSTEM
G asoline Engine
Two different types of air management systems are
used on light duty trucks.
An air injection reactor (AIR) (Fig. 6E-4) is used on
some engines to provide additional oxygen to continue the
combustion process after the exhaust gases leave the
combustion chamber. A belt driven air pump provides
pressurized air which is injected into the exhaust port of the
cylinder head or exhaust pipe and then into the exhaust
system. The AIR system operates at all times and will
bypass air only for a short duration of time during
deceleration and at high speeds. A diverter valve performs a
bypass function, and the check valve protects the air pump
from damage by preventing a back flow of exhaust gas.
Diagnosis and service procedures for air injection
reactor systems are in this section.
The Pulse Air Injection Reactor System consists of
four pulse air check valves. The check valves are connected
by tubes to the exhaust ports. The firing of the engine
creates a pulsating flow of exhaust gases which are positive
or negative pressure. A negative pressure at the pulse air
valves results in the flow of fresh air into the exhaust system.
If pressure is positive the check valve is forced closed and no
exhaust gas will flow past the valve into the fresh air supply
line.
A deceleration valve is used on all L6 and automatic
V8 models to prevent backfiring in the exhaust system
during decleration. When deceleration causes a sudden
vacuum increase in the vacuum signal line, the pressure
differential on the diaphragm will overcome the closing
force of the spring, opening the valve and bleeding air into
the intake manifold.
Air trapped in the chamber above the vacuum
diaphragm will bleed at a calibrated rate through the delay
valve portion of the integral "Check and Delay Valve, "
reducing the vacuum acting on the diaphragm. When the
vacuum load on the diaphragm and the spring load become
equal, the valve assembly will close, shutting off the air flow
into the intake manifold.
The check valve portion of the "Check and Delay
Valve" provides quick balancing of chamber pressure when
a sudden decrease in vacuum is caused by accleration rather
than deceleration.
THER M O STA TIC AIR CLEANER (THERM A C)
G asoline Engine
The Thermostatic Air Cleaner (THERMAC) is on all
engines. The TAC uses a damper door in the air cleaner
inlet, controlled by a vacuum diaphragm motor to mix pre­
heated and non pre-heated air entering the air cleaner to
maintain a controlled air temperature into the carburetor.
The vacuum motor is modulated by a temperature sensor in
the air cleaner. The pre-heating of the air cleaner inlet air
allows leaner carburetor and choke calibrations resulting in
lower emission levels, while maintaining good driveability.
The pre-heated air is obtained by drawing inlet air through
stove attached to the exhaust manifold. Diagnostics and
service procedures for the thermostatic air cleaner can be
found in this section.
THROTTLE RETURN C O N TR O L (TRC) SYSTEM
G asoline Engine
The TRC system used on heavy duty emission vehicles
consists of three major components:
1. Throttle Lever Actuator - Mounted as part of the
carburetor assembly, this device opens the primary
throttle blades a preset amount in excess of curb idle
when engine vacuum is applied to it. This actuating
vacuum is controlled by a separate solenoid control
valve.
2. Solenoid Vacuum Control Valve - Mounted separately
from the carburetor, this off-on valve is held open above
a present nominal engine speed by a signal from an
electronic speed sensor. The valve when open allows a
vacuum signal to be applied to the throttle lever
actuator as long as the present engine speed is
exceeded.
3. Electronic Speed Sensor - Mounted separately from the
solenoid vacuum control valve, this switching