Exhaust Gas Recirculation (Egr) System; Description; Diagnosis - GMC G Service Manual

6E2-52 DIESEL EMISSIONS

EXHAUST GAS RECIRCULATION (EGR) SYSTEM

DESCRIPTION

The EGR system lowers the formation of nitrogen oxides
by reducing combustion temperature. This is done by intro­
ducing exhaust gases into the cylinders through an EGR
valve. The ECM, in the Diesel Electronic Control system,
controls the amount of EGR to meet emission control
requirements and maintains good driveability through an
EGR solenoid, which regulates the vacuum to the EGR
valve. Two main sensor inputs to the ECM are used to
calculate the amount of EGR.
The EGR valve (Figure 29), installed on the intake mani­
fold, introduces the exhaust gases to the incoming fresh air
at the engine crossover. The EPR valve (Figure 30),
installed between the exhaust manifold and the exhaust
pipe, is used to increase exhaust back pressure during idle,
which increases the exhaust flow through the EGR system.
The EPR valve is normally open.
A vacuum pump is required to provide a vacuum source
to operate the EGR system.
Engine Speed Sensor, mounted at the rear of the engine
is used as an input to the ECM to measure the rpm of the
engine.
Throttle Position Sensor (Figure 26), mounted to the
injection pump throttle valve, is used as an input to the ECM
to measure the degree of throttle angle.
MAP Sensor, mounted on the left side of the cowl is
used to measure the amount of absolute pressure in the
ECM vacuum line.
EGR/EPR SOLENOID ASSEMBLY
The EGR/EPR Solenoid Assembly (Figure 31) is
mounted on top rear of the engine. The ECM controls the
EGR solenoid to regulate the vacuum to the EGR valve. By
controlling the time the EGR solenoid is "ON" or "OFF"
regulates the amount of EGR. The ECM calculates the
amount of EGR based on inputs from the engine speed
sensor and the throttle position sensor. The ECM is pro­
grammed to vary the "ON" and "OFF" time of the EGR
solenoid, based on these two sensor inputs. To monitor the
ECM control of EGR, a MAP sensor is used to measure the
amount of absolute pressure in the EGR vacuum line. If a
minor variation if calculated EGR and actual EGR as moni­
tored by the MAP sensor exists, the ECM makes a correc­
tion. If the variation exceeds an amount in excess of what
the ECM can correct for, an error is detected by the ECM
and the system will go into default.
When the ECM recognizes the operating range for no
EGR, the EGR vent solenoid operates to allow rapid vent­
ing of vacuum to the EGR valve. The ECM signal energizes
the EPR solenoid at idle, which allows vacuum to close the
EPR valve.
fTl
EPR VACUUM ACTUATOR
Figure 30 — EPR Valve

DIAGNOSIS

The diagnosis of the EGR system is part of the Diesel
Electronic Control system and starts with the Diesel
8S 3962-6E