Positive Back Pressure EGR Valve
A small diaphragm control valve inside EGR valve assembly acts as a pressure regulator. Control valve receives an exhaust back pressure signal through a hollow shaft, which exerts a force on bottom of control valve diaphragm, opposed by a light spring. A metal deflector plate prevents hot exhaust gases from flowing directly onto diaphragm. See Fig 1 .
Vacuum is applied to EGR valve assembly from carburetor spark port, to assure no exhaust gas recirculation at idle. During off-idle operation, manifold vacuum is applied to vacuum chamber through a restriction in signal tube.
When engine load is light, and back pressure is low, control valve is open. This allows air to flow from 6 bleeds in diaphragm plate, through control valve orifice, into vacuum chamber. Air bleeds off vacuum, decreasing signal trying to open EGR valve. Therefore, if back pressure does not close control valve, sealing off air flow, there will be no vacuum built up to open EGR valve for exhaust gas recirculation.
When power demands are made on engine, and exhaust gas recirculation is needed, exhaust gas back pressure increases, closing control valve. This shuts off air flow through valve. Vacuum builds up in vacuum chamber, until spring force holding EGR valve closed is overcome.
Once EGR valve opens, exhaust pressure decreases, because some of the exhaust gas is flowing into intake manifold through EGR passage. In actual operation, system will reach a balanced condition, providing optimum EGR operation.
Any increase in engine load will momentarily increase exhaust signal, causing control valve to close. This allows a stronger vacuum signal. The system will then stabilize at a greater EGR flow.
At maximum engine load, when manifold vacuum is nearly zero, momentarily, there will be no EGR operation. This is because of insufficient vacuum to pull valve open, even though high exhaust back pressure has closed control valve.