Exhaust Gas Recirculation (Egr) - Craftsman 20930 Operator's Manual

EXHAUST GAS RECIRCULATION (EGR)

An Exhaust Gas Recirculation (EGR) system is used
on most modern engines to reduce Oxides of
Nitrogen (NOx) emissions. During the combustion
process, nitrogen, which makes up 80 percent of
the air, will mix with oxygen at temperatures
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 6) may be
operated by ported vacuum from above the
throttle plates, or by a sophisticated control
system that modulates 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 vacuum. At idle and wide-open throttle, the
ported vacuum supply is low and the valve should
be closed.
No Vacuum Signal
Valve closed, exhaust blocked
FIGURE 6: EGR VALVE OPERATION
To Vacuum Source
Vacuum Signal Applied
Valve open, exhaust admitted to intake manifold
9
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 available.
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 normally with the engine
warm but remain open when the engine is cold.
That condition could be caused by a faulty thermal
switching device that does not cut off the vacuum
supply when the engine is cold.
To Vacuum Source