Craftsman 20930 Operator's Manual page 14

EXHAUST GAS RECIRCULATION (EGR)
EGR VENTURI VACUUM AMPLIFIER
Some engines utilize a Venturi Vacuum Amplifier
that uses the weak vacuum signal from the throat of
the carburetor to allow the passage of the stronger
intake manifold vacuum to operate the EGR valve.
On most applications the amplifier provides a 2" Hg
boost to the Venturi signal (FIGURE 8).
SERVICE PROCEDURES
1) Start the engine, and run it at idle until it reaches
normal operating temperature.
2) Make sure the intake manifold hose to the
amplifier is properly connected. On those systems
with a reservoir, remove the hose from the reservoir
and use a tee connector to join the hose to the
intake manifold vacuum hose.
3) With separate lengths of hose and different
connectors, bypass any and all vacuum valves or
coolant controlled valves between the amplifier
and the EGR valve.
4) Use a tee connector to attach the pump into the
vacuum line between the amplifier and EGR valve.
5) Increase engine speed to 1500 to 2000 RPM
and release the throttle. Let the engine return to
idle speed and remove the vacuum hose at the
carburetor venturi. The vacuum reading should be
within ± 0.3" Hg of the specified boost for that
amplifier if other than zero boost is specified.
Zero boost may read from 0 to .5" Hg. Replace
amplifier if it is out of specification.
6) Increase engine speed. Watching the vacuum
gauge, release the accelerator after a speed of
1500 to 2000 RPM is reached. If the vacuum gauge
reading shows an increase greater than 1" Hg
during acceleration period, the amplifier should
be replaced.
7) Remove the pump from the output vacuum line
and reconnect the hoses, but still bypass other
valves. Connect the pump and apply 2 to 4" Hg of
vacuum to port on the amplifier which is normally
connected to intake manifold vacuum. The EGR
valve should operate and engine idle should drop or
become erratic. If the EGR valve fails to move,
replace the amplifier.
BACK-PRESSURE TRANSDUCER VALVE
(BPV) OPERATION
The Back-pressure Transducer Valve (BPV) controls
the amount of EGR according to the load on the
engine. An exhaust pressure probe extends into
the exhaust crossover passageway to sample the
exhaust gas pressure. During light engine loads,
the pressure in the exhaust passageway is relatively
low, while during wide-open throttle operation
(WOT), the pressure is highest. This pressure signal
is transmitted to a diaphragm in the BPV and is used
to control the amount of vacuum applied to the EGR
valve (FIGURE 9).
SERVICE PROCEDURES
1) Remove the air cleaner and plug the intake
manifold fitting. Start the engine and bring it to
normal operating temperature. Position the fast-idle
cam follower on the second step of the fast-idle cam
(to obtain about 1500 RPM), and then note engine
speed on a tachometer. Use the pump to check the
source vacuum at an intake manifold port (FIGURE
10). Note this reading.
2) Tee your pump into the vacuum passageway to
the BPV and the reading should be 1 to 2" Hg of vac-
uum. Replace the BPV if it is not within specifications.
3) Leave the vacuum gauge at this location, remove
the hose to the EGR valve, and plug the hose open-
ing. Read the vacuum pump gauge, which should be
the same as the intake manifold vacuum reading. If
it is not within 2" Hg of the source vacuum, replace
the BPV valve.
To Distributor
Spark–
EGR Thermal
Vacuum Valve
FIGURE 9:
EXHAUST BACKPRESSURE TRANSDUCER VALVE
12
To EGR
Valve
Exposed to Exhaust Gas Pressure