My W211 OM642 had a good bit of carbon buildup on the firewall heat shield as well. I cleaned it off and it doesn't appear to be returning too quickly, but good to know about the flex pipe. I also wondered if a leaking turbo inlet seal could be contributing. Mine was very old and was not sealing properly, but it has since been replaced.

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2009 E320 Bluetec - 242k miles and a work in progress!