While at 400k+ miles, it certainly does not owe you anything!

We need more info:

New or old style vacuum pump?

Has head ever been off?

What are operating temps at?

If you have a way to check boost, what are you getting? ("T" in from the line coming from intake manifold)

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000