If you have never cleaned it I'll bet you a cold one (when I am in the area or you are in mine) your egr line is plugged from the egr valve to the manifold. On the E320 the check engine light comes on whenever this happens. I used an old speedo cable to rotorooter it out and when I replaced the head gasket a month or so ago I thoroughly cleaned it out. The majority of the pluggage will be at the manifold end. Also, on the E320 the tube inserts into the manifold about an inch or so past the point that it bolts up. In talking with Scott P at the last ATL g2g he said that they usually just replace this line with a new one. The rotorooter trick does not clean it out completely and the problem comes back too soon. You should be able to ascertain if the egr valve is inoperative by doing a bench test on it. Put the vacuum to it and see if you can blow through it.
I have considered modifying my egr pipe with a straight "clean out" at the manifold end with something I can easily remove/replace - something like a plumbing clean out.

Oh yea - the recirculated exh gas is used to drive the temp down which results in reduced NOX emissions. The same technology is used on steam boilers.

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Jim