Auto Parts: 1985 280SL, exhaust gas recirculation valve, digital fuel injection

Question
What is an EGR? It is located right on the side of the motor.

Answer
    An EGR is an Exhaust Gas Recirculation valve.  They were developed in the early '70's as a way to keep engines from pinging.  In order to pass the emissions tests that were mandatory from '69 on the manufacturers rigged the engines to run a little lean under part-throttle cruise.  Since this is where the engine spends most of its operating life it made a big dent in the emissions output.  But a lean mixture burns hot and can cause detonation or pinging.  This can be very bad for the engine, it breaks ring lands and burns holes in pistons.  So, to prevent this from happening they built a little gizmo that, under the right load conditions, leaks a little bit of the exhaust back into the intake tract to mix with the intake charge of gasoline and air.  Now, here's the hard to understand part:  leaking hot exhaust gas into the cylinder cools the mixture.  It does this because, when the mixture is ignited, the exhaust gas won't burn.  It's already been burned.  The unburned gas in the cylinder lowers the temperature of the charge and stops the pinging.  By the late '80's they had begun to use very advanced digital fuel injection and were able to accomplish the same thing via super-accurate control of the mixture and spark.  You still see EGR's on vehicles into the late '90's, especially Ford trucks, they are very cheap to make and very reliable until they wear out.