Mityvac 06820 Diesel Compression Test Adapters - Manual (Page 15)

Page Number - 15
Form 822378
BACK-PRESSURE TRANS-
DUCER 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
OPERATION
Spark Delay Valves (SDV) are used to
delay vacuum to the distributor vacuum
advance actuator during hard accelera-
tion, to delay the action of the Thermactor
Air Induction Reaction (AIR) system
during prolonged engine idling, and to
delay the application of vacuum to the
automatic choke pulldown diaphragm
during cold engine operation.
A sintered metal valve is installed in the
vacuum advance (outer) diaphragm of
the distributor control unit on some
engines. The purpose of the valve is to
delay the spark advance during rapid
acceleration to minimize the formation of
NOx. The sintered metal is porous and
allows vacuum to bleed through the valve
acting like an orifice of about 0.002” in
diameter. Control is obtained by varying
the number of discs in each valve
assembly so that the time delay features
can be tailored to the engine (FIGURE 11).
SERVICE PROCEDURES
The time delay of the valve varies with
engine application. The different valves
may be identified by color and part
number. Spark delay valves cannot be
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, and 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 pas-
sageway to the BPV and the reading
should be 1-2” Hg of vacuum. Replace
the BPV if it is not within specifica-
tions.
3) Leave the vacuum gauge at this
location, remove the hose to the EGR
valve, and plug the hose opening.
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.
SPARK DELAY VALVE (SDV)
repaired and must be replaced every
12,000 miles because the pores of the
sintered metal fill with dust, which can
slow the performance of the valve.
NOTE: The spark delay valve is a one-
way unit that must be installed with the
Black side facing the carburetor vacuum
port.
To determine if a spark delay valve is
operating correctly, the following service
procedure should be used:
1) With the transmission in neutral, set
the carburetor to the fast-idle
position, remove the spark-delay
valve and tee your vacuum pump into
the hose leading to the carburetor
spark port.