I'm not 100% certain but I think this can also refer to the butterfly valve which closes the inlet manifold when the DPF is regenerating, and when you switch the engine off (to stop the engine shuddering).
This gets a lot of oily goop over time.
If you remove the big inlet hose from the intercooler (1 jubilee clip), the flap is right there just inside the manifold. Give it a spray with some carb cleaner, then let it all evaporate before you reconnect the hose.
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