Form 824426
Page Number - 13
An Exhaust Gas Recirculation (EGR) sys tem is used
on most modern engines to reduce Oxides of
Nitrogen (NOx) emis sions . During the combustion
process, nitrogen, which makes up 80 percent of
the air, will mix with oxygen at tempera tures
above 2,500°F . During the combustion process,
temperatures in the cylinders go well above
3,500°F providing the ideal conditions for the
formation of NOx .
SYSTEM OPERATION
To reduce the formation of NOx, it is necessary to
lower the combustion temperature . This is most
often done by introducing exhaust gases back into
the combustion chamber through the use of an
EGR valve . The EGR valve (FIGURE 8) may be
operated by ported vacuum from above the
throttle plates, or by a sophisticated control
system that modu lates the amount of EGR
depending on the temperature of the coolant,
ambient air temperature, engine speed or load .
An EGR valve that does not have a sophisticated
control system must be fully closed with a vacuum
of less than 2” Hg and begin to open with 2 to 8 .5”
Hg of vacu um . At idle and wide-open throttle, the
ported vacuum supply is low and the valve should
be closed .
Some cars have a Back-Pressure Transducer Valve
(BPV) to modulate the operation of the EGR system .
Some cars have a Venturi Vacuum Amplifier (VVA)
to do the same job . The effect is to modulate the
amount of EGR according to the load on the engine .
To improve cold drivability, most cars are equipped
with some type of vacuum control device to shut off
EGR while the engine is cold .
EGR systems fail in two ways . Either the valve may
fail due to a fault of its own, such as a ruptured
diaphragm, or due to a loss of control vacuum .
Always check whether there is vacuum at the hose
connected to the EGR valve, before replacing the
valve . Connect the pump to the vacuum supply hose
at the EGR valve and check whether at 2000 RPM
there is at least 4 to 5” Hg vacuum avail able .
Remember also that clogged exhaust passages that
lead to or from the valve can restrict the flow even
if the valve is opening .
An EGR valve that remains open will cause the
engine to idle roughly, die at idle, and lose power
and full-throttle smoothness . Dirt or damage in the
valve seat area usually cause the valve to fail . An
EGR valve can operate nor mally with the engine
warm but remain open when the engine is cold .
That condi tion could be caused by a faulty thermal
switching device that does not cut off the vacuum
supply when the engine is cold .
eXhaust Gas recircuLation (eGr)
FIGURE 8: EGR VALVE OPERATION
To Vacuum Source
To Vacuum Source
Vacuum Signal Applied
Valve open, exhaust admitted to intake manifold
No Vacuum Signal
Valve closed, exhaust blocked
