Mityvac MV8000 Mode D'emploi page 14

EXHAUST GAS RECIRCULATION (EGR)
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.
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
Ported Vacuum
FIGURE 8: GM EGR DIAPHRAGMS
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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 8). 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.
2) Check the vacuum connection to the EGR valve
for obstructions.
Positive Backpressure Negative Backpressure
Form 824345 Form 824345