The EGR (Exhaust Gas Recirculation) concept was imposed upon many American cars in the 1970's. It's purpose was to reduce combustion temperature in the cylinders to reduce the creation of incomplete nitrous oxides, such as NO and NO2. It accomplished the lowering of combustion temperature by reducing the efficiency of the burning of the air-fuel mix. Great idea, huh?
Then, on carbureted (gasoline) engines, the AIR (Air Injection Reactor, GM's name for it) system, which actually appeared in the late '60's, took some of the air-fuel mix from under the carburetor (American spelling), mixed it with air coming in through a belt driven pump, and forced this mix into the exhaust coming out of the engine, usually right into the exhaust manifold. This was to reduce carbon monoxide and unburned hydrocarbon emissions coming from the engine. This is particularly "useful" in reducing the elevated level of unburned hydrocarbons created by the EGR system. For older cars without oxygen sensors and computers that use them, this crap can be removed for "testing" and greatly improves miles per gallon and power. For newer cars with such sensors and computers, your stuck with keeping them so as not to throw the computer out of kilter.
The EGR system is bad enough on a gasoline engine, it's hell on a diesel. I think most of our older diesels can safey be "tested" without an EGR system indefinitely.
John