All 1973 models are equipped with an exhaust gas recirculation (EGR) system to control oxides of nitrogen.
On V8 engines, exhaust gases travel through the exhaust gas crossover passage in the intake manifold. On 6 cylinder engines, an external tube carries exhaust manifold gases to a carburetor spacer. On spacer entry equipped engines, a portion of these gases are diverted into a spacer which is mounted under the carburetor. On floor entry models, a regulated portion of exhaust gases enters the intake manifold through a pair of small holes drilled in the floor of the intake manifold riser. The EGR control valve, which is attached to the rear of the spacer or intake manifold, consists of a vacuum diaphragm with an attached plunger which normally blocks off exhaust gases from entering the intake manifold.
On all models, the EGR valve is controlled by a vacuum line from the carburetor which passes through a ported vacuum switch. The EGR ported vacuum switch provides vacuum to the EGR valve at coolant temperatures above 125°F. The vacuum diaphragm then opens the EGR valve permitting exhaust gases to flow through the carburetor spacer and enter the combustion chambers. The exhaust gases are relatively oxygen-free, and tend to dilute the combustion charge. This lowers peak combustion temperature thereby reducing oxides of nitrogen.
On some models equipped with a 351C V8, an EGR subsystem, consisting of a speed sensor and control amplifier, prevents exhaust gases from entering the combustion mixture when the car is traveling 65 mph or faster.