Exhaust Emissions System - Exhaust Gas RECIRCULATION: Operation
On 1975-77 models, the EGR valve is controlled by vacuum. When valve opens, a small quantity of exhaust gases flow through crosspipe and EGR valve into intake manifold. When intake air passes through venturi, a vacuum signal, which is proportional to total air flow, is obtained. This vacuum signal is transmitted to vacuum amplifier which amplifies signal by drawing vacuum from vacuum reservoir.
The amplified signal then goes to EGR valve through the vacuum signal switch and PVS valve. Vacuum signal switch cuts out EGR vacuum signal at engine speeds below 2000-3000 RPM. The PVS also senses coolant temperature and cuts out EGR vacuum signal at coolant temperatures lower than 100°F (38°C). At wide open throttle the EGR valve is closed.
On 1978-79 models, the EGR valve is controlled by vacuum. When valve opens, a small quantity of exhaust gases flow through crosspipe and EGR valve into intake manifold. As throttle valve opens slightly and passes the two vacuum ports, a gradual increase in vacuum occurs, which opens the EGR valve. However, at wide open throttle, the vacuum decreases, but normal vacuum level is maintained for 6 seconds by the holding valve.
As throttle valve closes, the EGR valve must also close to prevent rough idle. The release valve bleeds off vacuum so that EGR valve closes. See Fig 1. Below 104°F (40°C) coolant temperature, the PVS will shut off vacuum to the EGR valve to maintain driveability while engine is cold.