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TM 9-8000
The crossover passage in the intake
manifold is used as the source of exhaust gas for the
system.  A vacuum-operated EGR valve controls the
passage of exhaust gas to the engine intake. Exhaust
gas recirculation will occur whenever vacuum is applied
to the valve.
(2) The EGR valve should not be operational
before the engine reaches operating temperature. If it
were, it would cause extreme drivability problems. Also,
there is no reason for exhaust gas recirculation before
warmup because virtually no oxides of nitrogen are
manufactured.  The EGR valve also will not operate
during periods of heavy engine loading because it would
hinder performance.
There are two different methods of
supplying vacuum to the EGR valve.
(a)  One system uses a vacuum port into the
Figure 7-10. Naturally Aspirated System.
carburetor throat located just above the throttle plate. As
the throttle begins to open, vacuum will begin to be
air naturally into the exhaust system.  The primary
applied to the port and operate the EGR valve.  The
purpose of this system is to provide the necessary
valve will continue to operate fully until approximately half
oxygen to the catalytic converter.
throttle is reached.  As the throttle is open past the
halfway point, exhaust gas recirculation gradually will
(1)  The key to the aspirator system Is the
diminish to zero as the throttle approaches the fully
aspirator valve. It is basically a one-way check valve that
opened position.
allows air to be drawn into the exhaust system during
(b)  Another system uses a vacuum port that Is
negative pulses, yet blocks the exhaust from passing out
directly in the carburetor venturi.  This will provide
through the valve.
vacuum for the EGR valve whenever the engine Is
running at high speed. The problem with using venturi
(2) The aspirator valve connects to the air filter
vacuum is that it is not strong enough to operate the
housing by a rubber hose to take In air. The aspirator
EGR valve.  To make the system feasible, manifold
valve connects to the exhaust through the air manifold.
vacuum is used to operate the EGR valve through a
7-10. Exhaust Gas Recirculation System (Fig. 7-11).
vacuum amplifier.  The vacuum amplifier switches the
manifold vacuum supply onto the EGR valve whenever
a.  Purpose.  When  the temperature of the
venturi vacuum Is applied to its signal port. At times of
combustion  flame  exceeds  approximately  2500F
large engine loading (wide throttle opening), manifold
(1370C), the nitrogen that is present in the Intake air
vacuum will be weak, producing the desired condition of
begins to combine with the oxygen to produce oxides of
no exhaust gas recirculation.
nitrogen (NOx).  The exhaust gas recirculation (EGR)
system helps to control the formation of oxides of
(4) A switch that uses coolant temperature as
nitrogen by recirculating a portion of the exhaust gases
an Indicator is used to block vacuum to the EGR valve
back through the Intake manifold, resulting in cooler
before the engine reaches operating temperature. This
combustion chamber temperatures.
valve is used with all systems.
b. Operation. Though there are minor differences
between systems, the basic operation of an exhaust gas
recirculation system is as follows:


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