Steve Brotherton wrote an article and posted pictures on the blockage of the hot inlet pipe at the intake manifold. Here is alink to the article. It is posted on the DIY link at the top of the page:
Steve Brotherton and Benzmac have posted that they perform the cleaning of this pipe every 60,000 miles.
I have also read a post by Benzmac that he also installs a new pipe these days as it is cheaper to do so than to take the time to clean it out.
There is a lot in the archives on cleaning out this pipe if you try the search function.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".