EGR Valve

The EGR valve is mounted on the side of the intake manifold on 6-258 engines.

Fig. 22 EGR valve without exhaust back pressure sensor:

Fig. 23 EGR CTO switches:

Fig. 24 EGR valve with integral exhaust back pressure sensor:

The EGR valve without an exhaust back pressure sensor, Fig. 22, is normally held closed by a spring located above the diaphragm. The valve opens when enough vacuum is applied through the hoses connecting the coolant temperature override (CTO) switch, Fig. 23, to the EGR vacuum port at the carburetor. When vacuum overcomes the spring tension, a pintle within the valve is lifted off its seat and exhaust gas is metered into the intake manifold.
The EGR valve with an integral exhaust back pressure sensor, Fig. 24, combines the functions of the EGR valve and separate back pressure sensor into a single unit. This unit is calibrated by the use of selective diaphragm spring loads and flow control orifices. The exhaust gas flow is controlled by a moveable pintle. When no vacuum is applied to the diaphragm, the pintle is held against its seat by spring tension, confining exhaust gases to the exhaust manifold. When vacuum is applied to the diaphragm, the pintle is forced from its seat, however, this cannot occur if the vacuum bleed valve in the power diaphragm is opened. Exhaust gas back pressure from inside the exhaust manifold flows through the hollow pintle stem into the control diaphragm chamber. If this pressure is enough to overcome the control spring tension, the control diaphragm moves against its bleed valve. Full vacuum is now applied to the power diaphragm and the pintle moves, allowing exhaust gas to flow to the intake manifold. If the exhaust back pressure drops enough, the control diaphragm moves away from the bleed valve and the pintle returns to its seat.