I'm starting to suspect a partially clogged EGR valve as well. I guess going from 20" to 17"Hg intake vacuum when the EGR opens is not considered enough of a change for the OBD to say that the EGR system is functioning correctly. You suggested detaching the EGR pipe and then checking manifold vacuum. Would this be comprable to running the engine with the EGR valve open?

Finally, disconnecting the pipe on this 1996 SL320 looks like a difficult job. The pipe wraps around the back of the engine and i think it has to be completly removed in order to clean it. Too risky cleaning it attached on the manifold side with the EGR side opened cause loosened debris could gert sucked into the engine. If I take the plunge are there any gaskets etc I may need when putting it back together?

Did I understand you correctly when you wrote:
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I think engine manifold vacuum would get weaker with the EGR valve pipe disconnected? if the vacuum goes much lower than 17" would that suggest a partially clogged pipe.

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1972 450SL
1982 300D Turbo