A basic EGR system is simple, consisting of a
vacuum operated EGR valve and a vacuum line from
the carburetor. The EGR valve usually bolts to the
engine intake manifold or a carburetor plate. Exhaust
gases are routed through the cylinder head and intake
manifold to the EGR valve.
The EGR valve consists of a vacuum diaphragm, a
spring, an exhaust gas valve, and a diaphragm housing.
It is designed to control exhaust flow into the intake
Though there are minor differences between
systems, the basic operation of an exhaust gas
recirculation system is as follows:
At idle, the throttle plate in the carburetor or fuel
injection throttle body is closed. This blocks off engine
vacuum, so it cannot act on the EGR valve. The EGR
spring holds the valve shut, and the exhaust gases do
NOT enter the intake manifold. If the EGR valve were
to open at idle, it could upset the air-fuel mixture and the
engine would stall.
When the throttle plate is swung open to increase
speed, engine vacuum is applied to the EGR hose.
Vacuum pulls the EGR diaphragm up. In turn, the
diaphragm pulls the valve open. Engine exhaust can
enter the intake manifold and combustion chambers. At
higher engine speeds, there is enough air flowing into
the engine that the air-fuel mixture is not upset by the
open EGR valve.
There are two different methods of supply vacuum
to the EGR valve as follows:
The first method uses a vacuum port into the
carburetor throat located just above the throttle plate.
As the throttle begins to open, vacuum will begin to be
applied to the port and operates the EGR valve. The
valve will continue to operate fully until approximately
half throttle is reached. As the throttle is open past the
halfway point, exhaust gas recirculation gradually will
diminish to zero, as the throttle approaches the fully
The second method uses a vacuum port that is
directly in the carburetor venturi (fig. 4-53). The
carburetor venturi provides vacuum for the EGR valve
any time the engine is running at high speed. The
problem with using venturi vacuum is that it is not
strong enough to open the EGR valve. So to make it
work, manifold vacuum is used to operate the EGR
valve through a vacuum amplifier. The vacuum
amplifier switches the manifold vacuum supply to the
EGR valve whenever venturi vacuum is applied to its
signal port. At times of large engine loading (wide,
open throttle), manifold vacuum will be weak,
producing the desired condition of no exhaust gas
An engine coolant temperature switch may be used
to prevent exhaust gas recirculation when the engine is
cold. A cold engine does not have extremely high
combustion temperatures and does not produce very
much NOx. By blocking vacuum to the EGR valve
below 100°F, you can improve the drivability and
performance of the cold engine.
FUEL EVAPORIZATION CONTROL
The fuel evaporization control system prevents
vapors from the fuel tank and carburetor from entering
the atmosphere (fig. 4-54). Older, pre-emission
vehicles used vented fuel tank caps. Carburetor bowls
were also vented to the atmosphere. This caused a
considerable amount of emissions. Modern vehicles
commonly use fuel evaporization control systems to
prevent this source of pollution. The major components
of the fuel evapotization control systems are the sealed
fuel tank cap, fuel air dome, liquid-vapor separator, roll-
over valve, fuel tank vent line, charcoal canister,
carburetor vent line, and the purge line.
SEALED FUEL TANK CAP is used to keep fuel
vapors from entering the atmosphere through the tank
filler neck. It may contain pressure and vacuum valves
that open in extreme cases of pressure or vacuum.
When the fuel expands (from warming), tank pressure
forces fuel vapors out a vent line or line at the top of the
fuel tank, not out of the cap.
FUEL AIR DOME is a hump designed into the
top of the fuel tank to allow for fuel expansion. The
dome normally provides about 10 percent air space to
allow for fuel heating and volume increase.
LIQUID-VAPOR SEPARATOR is frequently
used to keep liquid fuel from entering the evaporation
control system. It is simply a metal tank located above
the main fuel tank. Liquid fuel condenses on the walls
of the separator and then flows back into the fuel tank.
ROLL-OVER VALVE is sometimes used in the
vent line from the fuel tank. It keeps liquid fuel from
entering the vent line after an accident where the vehicle
rolled upside down. The valve contains a metal ball or
plunger valve that blocks the vent line when the valve is