Chapter
D
: Emission Control Systems Diagnosis and Repair
76
systems operate only when there is a slight positive
pressure in the exhaust system. These have an in-
tegral or external backpressure transducer. It may
be necessary to restrict the exhaust pipe when ap-
plying vacuum to the EGR valve to make it operate.
If there was no vacuum signal to the valve, test the vac-
uum circuit back to the source. If the vacuum is con-
trolled by the PCM, certain conditions need to be met be-
fore the EGR valve is activated. Typically, these include
most or all of the following:
• Engine coolant is at or above a minimum tempera-
ture
• Engine speed is above idle and below a maximum
specified RPM
• The vehicle is moving at a minimum speed
• The control system is in closed-loop operation
• The throttle is open, but not wide open
• There is backpressure in the exhaust
Exact test conditions and specifications vary. Follow the
Service Manual procedure for the particular system.
EGR Valve Replacement
Most EGR valves bolt to the intake manifold or cylinder
head. Simply remove the bolts and lift off the valve. In-
spect the valve and manifold passages for combustion
deposits. These deposits are a normal result of contin-
ued operation. Excessive buildup can clog exhaust gas
passages in the intake manifold and cause a new valve
to fail.
Some EGR valve assemblies can be cleaned and rein-
stalled. Other valve designs cannot be cleaned and must
be replaced. When cleaning a valve, be careful not to
damage the diaphragm.
Install the EGR valve using a new gasket, and tighten
the fasteners to the specified torque value to prevent ex-
haust gas leaks.
EGR Vacuum Controls
The vacuum signal to the EGR valve may be regulated
by one or more of the following:
• Vacuum amplifier
• Temperature control valve
• Thermal vacuum switch
• Time delay solenoid
• Backpressure transducer
• Pulse width modulated (PWM) solenoid
Many components of vacuum operated systems are
tested using a vacuum pump while electronically con-
trolled systems require the use of a scan tool and digital
mulitmeter. Test procedures vary; accurate service infor-
mation is required. Make sure all EGR connections are
properly routed and tight. Replace any hoses that are
pinched, cracked, broken, or otherwise damaged.
Not all vehicles rely on an EGR valve to control NO
x
emissions. Many late-model vehicles do not require the
valve or external systems to keep the emissions within
acceptable limits.
Fig. 4-6.
Typical stepper motor controlled EGR valve.
TO VACUUM SOURCE
TO VACUUM SOURCE
EXHAUST GAS
NO VACUUM SIGNAL
VACUUM SIGNAL APPLIED
EXHAUST ADMITTED
TO INTAKE MANIFOLD
CLOSED
VALVE
OPEN
VALVE
POPPET
VALVE
POPPET
VALVE
Fig. 4-5.
Simple vacuum-operated, spring-loaded diaphragm type
EGR valve.
Summary of Contents for ASE-A8
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