Mityvac MV8500 silverline elite aUtoMotive test kit - Manual (Page 14)

EGR SERVICE PROCEDURES/GENERAL TEST
EXCEPT GM OR BACKPRESSURE
CONTROLLED TYPE
If the symptoms of an engine lead you to believe
that an EGR valve is staying open, follow this
procedure:
1) Connect a tachometer to the engine and run
the engine at idle speed until it reaches normal
operating temperature . Use the pump to check for
at least 10” Hg vacuum at the valve . Replace the
hose and note the engine RPM .
) 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 000 RPM . Plug the
vacuum supply hose . Connect the vacuum pump to
the EGR valve and apply 10 to 15” Hg vacuum .
) 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 .
GM EGR VALVES
General Motors produces three types of EGR valves .
Each valve can be identified by the design of its
diaphragm plate (FIGURE 9) . 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 backpressure 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-PRESSURE EGR TEST
1) Make sure all vacuum hoses are routed
according to the emission control label .
) Check the vacuum connection to the EGR valve
for obstructions .
eXhaust Gas recircuLation (eGr)
FIGURE 9: GM EGR DIAPHRAGMS
Ported Vacuum Positive Backpressure Negative Backpressure
Page Number - 14 Form 8433