Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 389
Copyright by Goodheart-Willcox Co., Inc.
from the ECM. If the solenoid is receiving the close signal
and still allows vacuum to reach the EGR diaphragm, it is
defective. If the light does not come on, the problem is in
the ECM.
Checking EGR Position Sensors
If an EGR valve is equipped with a position sensor, the
sensor operation can often be checked with the scan tool as
the engine operates. This is done by monitoring the voltage
or percentage, Figure 18-22. Another common test of EGR
position sensors is to apply vacuum to the diaphragm with
a vacuum pump while observing voltage or resistance with
a voltmeter. Check the service information for exact test-
ing procedures and specifi cations. Generally, the electrical
reading should change as vacuum is varied. If the sensor
fails to produce the proper electrical reading at a specifi c
vacuum, it should be replaced.
Electronic EGR Diagnosis
One or more electric solenoids or motors operate the
pintle of an electronic EGR valve. The ECM calculates the
needed movement from the signals received from the EGR
valve itself. The condition of the EGR valve can be read and
tested by a scan tool, Figure 18-23. Switch the scan tool
to the intrusive test mode and test each solenoid. As each
solenoid is energized, the engine will decrease in speed,
idle rough, possibly set a code for excessive knock, and
even stall. If engine operation does not change when an
EGR solenoid is energized, either the valve, ECM, or associ-
ated wiring is defective.
EGR Flow Rate Monitoring
The ECM maintains close control of engine operat-
ing parameters to reduce NO
X
formation. Therefore, the
EGR is opened as little as possible to improve driveability.
However, the OBD II system has a procedure for monitor-
ing the fl ow rate through the EGR. The scan tool can read
the airfl ow through the EGR valve.
On some vehicles, fl ow rate is calculated by measuring
the pressure drop across the EGR valve. This is done with a
sensor similar in design to a MAP sensor. On other vehicles,
the computer determines EGR fl ow rate by reading pintle
position, manifold vacuum, and airfl ow and calculates
actual fl ow from one or more of these readings.
Note: When servicing any electrically
controlled EGR system, make all electrical
tests according to procedures in the
service information. Failure to do so may destroy
one or more of the ECM output drivers.
Checking the Air Pump and Valves
To test the air pump, the engine must be running. The
pump should be quiet, with little, if any, noticeable noise
at idle. If the pump is noisy, make sure the noise is not
coming from the belt. The diverter valve may fl utter when
the engine is idled. This condition is normal.
Next, race the engine while observing the diverter
valve muffl er. A low-pressure gauge can be used to check
the pump output. Pressure should be about 1 psi (7 kPa). If
not, check the air fi lter for clogging. If the engine is raced
and then the throttle is suddenly released, air should be
heard and felt exiting through the diverter valve muffl er,
Figure 18-24. If air does not exit, the diverter valve may be
defective.
If no airfl ow is felt from the muffl er, remove a hose to
the air manifold pipes (before the switching valve, if the
system has one) and observe whether air is being pumped
from the hose outlet. If air is coming out of the hose, the
pump is working. If no air is coming out, the pump is not
delivering air. Also, remove and check the vacuum-signal
line to the diverter valve. There should be vacuum when-
ever the engine is running.
Note that on air pump systems with switching valves,
the output air is delivered to the exhaust manifold when the
engine is cold and to the converter when the engine is warm.
This can be confi rmed by removing a hose past the switch-
ing valve. Check valves can fail and allow exhaust gases to
enter the air pump and control valves. If any evidence of
exhaust gas buildup is found (exhaust deposits or corrosion)
check the operation of the check valves. Check all hoses,
lines, and manifolds for kinks, pinching, and leaks.
Newer vehicles have electrically controlled bypass and
diverter valves. In addition to the tests described above, the
air pump and valves can sometimes be tested with a scan
tool. The scan tool should be used to obtain trouble codes
and other air injection system information.
Note: Be sure to check the drive belt on the
air pump for tension and condition.
Figure 18-22. Scan tools can report EGR operation by monitor-
ing solenoid voltage or pintle position. Note that there is low
voltage and no pintle movement at idle.
EGR Position Sensor Operation
State Park, neutral and
drive at idle
Driving except at
idle
EGR pintle
(voltage)
EGR valve control
solenoid
EGR valve control
solenoid
0.5 volts 2.4–5 volts
EGR pintle
(percentage)
EGR valve pintle EGR valve pintle
0 percent 1–100 percent