The exhaust gas recirculation (EGR) system is used to reduce the amount of
nitrogen oxide (NOx) emission levels caused by combustion temperatures exceeding 816°C
(1,500°F). It does this by introducing small amounts of exhaust gas back into
the combustion chamber. The exhaust gas absorbs a portion of the thermal energy produced
by the combustion process and thus decreases combustion temperature. The EGR system
will only operate under specific temperature, barometric pressure, and engine load
conditions in order to prevent driveability concerns and to increase engine performance.
The engine control module (ECM) calculates the amount of EGR needed based on the following
inputs:
| • | The engine coolant temperature (ECT) sensor |
| • | The intake air temperature (IAT) sensor |
| • | The barometric pressure (BARO) |
| • | The manifold absolute pressure (MAP) sensor |
| • | The throttle position (TP) sensor |
EGR Valve Circuits
The exhaust gas recirculation (EGR) valve consists of the following circuits:
| • | An ignition 1 voltage circuit which supplies 12 volts to
the coil of the EGR valve |
| • | A control circuit which grounds the coil of the EGR valve--The control
circuit is a pulse width modulated (PWM) ground produced by an internal low side driver
of the engine control module (ECM). |
| • | A 5-volt reference circuit supplied from the ECM to the internal position
sensor of the EGR valve |
| • | A signal circuit which sends a feedback voltage from the internal position
sensor of the EGR valve to the ECM--This voltage varies depending on the position
of the EGR valve pintle. The ECM interprets this voltage as the position of the EGR
valve pintle. |
| • | A low reference circuit supplied from the ECM to the internal position
sensor of the EGR valve |
EGR Diagnostics
The engine control module (ECM) tests the exhaust gas recirculation (EGR) flow
during deceleration by momentarily commanding the EGR valve to open while monitoring
the signal of the manifold absolute pressure (MAP) sensor. When the EGR valve is opened,
the ECM will expect to see a predetermined increase in MAP. If the expected increase
in MAP is not detected, the ECM records the amount of MAP difference that was detected
and adjusts a calibrated fail counter towards a calibrated fail threshold level. When
the fail counter exceeds the fail threshold level, the ECM will set a DTC.
Normally, the ECM will only allow one EGR Flow Test during an ignition cycle.
To aid in verifying a repair, the ECM allows extra EGR Flow Test during the first
ignition cycle following a code clear or a battery disconnect. The extra EGR Flow
Test should be sufficient for the ECM to determine adequate EGR flow and pass the
EGR flow test. If the ECM detects an EGR flow error, a DTC will set.
The ECM monitors the position of the EGR valve pintle via the EGR position sensor.
If the ECM detects a calibrated variance between the desired EGR valve pintle position
and actual position for a calibrated amount of time, a DTC will set.
The ECM also monitors the circuits of the EGR valve for electrical faults. If
a circuit fault is detected for a calibrated amount of time, a DTC will set.
