Mityvac 06820 Diesel Compression Test Adapters User Manual
Page 11
Page Number - 11
Form 822378
there is at least 4” to 5” Hg vacuum
available. Remember also that clogged
exhaust passages that lead to or from the
valve can restrict the flow even if the
valve is opening.
An EGR valve that remains open will
cause the engine to idle roughly, die at
idle, and lose power and full-throttle
smoothness. The valve usually fails to
close due to dirt or damage in the valve
seat area. An EGR valve can operate nor-
mally with the engine warm but remain
open when the engine is cold. That
condition could be caused by a faulty
thermal switching device that does not
cut off the vacuum supply when the
engine is cold.
SERVICE PROCEDURES -
GENERAL TEST EXCEPT
GM OR BACK-PRESSURE
CONTROLLED TYPE)
If the symptoms of an engine lead you to
believe that an EGR valve is staying
open, follow this procedure:
An EGR valve that does not have a
sophisticated control system must be
fully closed with a vacuum of less than
2” Hg and begin to open with 2-8.5” Hg of
vacuum. At idle and wide-open throttle,
the ported vacuum supply is low and the
valve should be closed.
Some cars have a Back-Pressure
Transducer Valve (BPV) to modulate the
operation of the EGR system. Some cars
have a Venturi Vacuum Amplifier (VVA) to
do the same job. The effect is to modulate
the amount of EGR according to the load
on the engine. To improve cold driveabili-
ty, most cars are equipped with some
type of vacuum control device to shut off
EGR while the engine is cold.
EGR systems fail in two ways. Either the
valve may fail due to a fault of its own,
such as a ruptured diaphragm, or due to
a loss of control vacuum. Always check
to be sure that there is vacuum at the
hose connected to the EGR valve, before
replacing the valve. Connect the pump to
the vacuum supply hose at the EGR valve
and check to be sure that at 2000 RPM