Exactly, which part was bad??
My '80 300D has a "heat control unit", $$$$, that is cracked, corroded, clogged and condemned. The PO installed a regular plumbing shut-off valve, which is not so good(a PITA).
A new unit, I think, is available for $700

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Adrian
'80 Mercedes 300D ..sold..
two Diesel VW caddies, old ones ..sold..
'96 Saab 900S