EGR System: Operation
Vacuum signal is supplied to EGR valve from a port upstream from throttle valve. When intake manifold temperature is below 86°F (30°C), vacuum signal is modulated through the orifice by an air bleed to the air cleaner (on EGR control valve orifice equipped models). Additionally, with intake manifold wall temperatures between 86°F (30°C) and 104°F (40°C), vacuum signal is further modulated through the thermo vacuum switch.
For Federal 2WD models the vacuum signal is also modulated with atmospheric pressure through a solenoid valve whose air flow is regulated by the electronic control module.
When EGR valve is open, exhaust gases flow through EGR valve and pipe and into the intake manifold. The strength of vacuum signal at EGR valve determines how much exhaust gas is recirculated.