The Exhaust Gas Recirculation (EGR) system was introduced on US-market gasoline engines in 1971 to reduce NOx emissions. However, it was not a standard feature on all vehicles, and its implementation varied between manufacturers and models.
While some 1978 350 Chevy engines might have had an EGR system installed, it was not a standard feature. To determine if your specific engine has an EGR valve, you should consult your vehicle's owner's manual, or refer to a repair manual specific to your model year and engine.
You can also try to visually locate the valve:
* Look for a small metal tube connected to the intake manifold. This tube will lead to the EGR valve.
* The EGR valve is typically located on the passenger side of the engine, near the intake manifold.
If you cannot locate the valve or are unsure, it's best to consult a qualified mechanic.
