Per the Factory Service manual, you would want to do a 'citric acid flush'. Its a pain, and a full day job.

Be warned though, with things that rusty, when you do the flush you may spring a leak in the radiator. Be ready to have the vehicle down for a few days and you credit card in hand :-)

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car