Here is something I found in an old posting. My engine stumbles as well so the pipe is not clogged?

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Arthur Dalton
Registered User Join Date: Aug 1999
Location: Florida
Posts: 2,887

Although there is a reasonable chance that the egr tube is blocked,[common
104 prob], an easy test for vac. flow is to simply apply some vac [ hand pump or vac source] to the egr while at idle. This opens the egr and will cause the engine to stumble..[ go lean]...if it does ,the tube is not blocked and other parts of the egr system are suspect...
You may know about the EGR/TSB on this chassis...[ archieves]
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1972 450SL
1982 300D Turbo