Since most Diesels have very poor intake vacuum, most systems have a vacuum pump driven from the engine. I would find if the engine has a vacuum pump and/or vacuum reservoir and make sure there is enough source vacuum.

I assume there should be source vacuum available on hose 15 in the diagram.

The more bandaids I see on these Diesels, they happier I am that I do not own one.

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2008 4.8i Black with Tabacco. Black Y-spoke wheels and shadowline trim.