Mityvac 06820 Diesel Compression Test Adapters - Manual (Page 12)

Page Number - 12
Form 822378
1) Connect a tachometer to the engine
and run the engine at idle speed until it
reaches normal operating tempera-
ture. Use the pump to check for at
least 10” Hg vacuum at the valve.
Replace the hose and note the engine
RPM.
2) Remove the vacuum hose from the
valve and notice whether engine RPM
increases.
3) If engine speed does increase, there
may be some type of problem in the
vacuum control circuit. Check the
routing of all vacuum hoses.
4) If engine speed or the quality of idle
changes, remove the valve and check
the pintle and valve seat to make sure
both are clean. If they are not, replace
the valve, gasket and adapter if it is
burned, warped or damaged.
If the engine symptoms lead you to
believe that the EGR valve is staying
closed, follow the procedure below:
1) Operate the engine at idle until it
reaches full operating temperature.
Use the pump to check for the
presence of 10” Hg vacuum at the
valve. Set the engine speed at
approximately 2000 RPM. Plug the
vacuum supply hose. Connect the
vacuum pump to the EGR valve and
apply 10” to 15” Hg vacuum. 2) The
diaphragm should move to the open
position and a decrease in engine
RPM should be noted. If not, the valve
is defective or the manifold passages
are plugged. Release the vacuum on
the EGR valve.
3) The diaphragm should move to the
closed position and an increase in
engine RPM should be noted. Return
the engine to idle and turn it off.
4) Connect the pump to the EGR valve
and test by applying at least 9, Hg of
vacuum to the diaphragm and watch
the gauge carefully for any vacuum
loss.
5) If the valve diaphragm does not move,
or cannot hold vacuum, replace the
EGR valve.
SERVICE PROCEDURES GM
EGR VALVES
General Motors produces three types of
EGR valves. Each valve can be identified
by the design of its diaphragm plate
(FIGURE 7). The first valve is a ported
vacuum EGR that has only a circular rib
on the back of its diaphragm plate. The
second is a positive back-pressure valve
with X-shaped ribs that are raised only
slightly above the plate. Finally, there is a
negative back-pressure valve with X-
shaped ribs raised well above the
diaphragm plate. Both the ported vacuum
and negative back-pressure valves are
tested the same way. A separate test is
listed to check the positive back-pressure
valve.
GM PORTED VACUUM AND
NEGATIVE BACK-PRES-
SURE EGR TEST
1) Make sure all vacuum hoses are
routed according to the emission
control label.
2) Check the vacuum connection to the
EGR valve for obstructions.
3) Connect the pump between the EGR
valve and the carburetor or vacuum
source. Start engine and run at the
idle until it reaches operating