I had a very similar situation with my daily driver ('84 190D); no trace of coolant in the oil - coolant down a pint or so every oil change (never more than that) - no indications of leaks anywhere, been that way for over three years and 150K miles. Until last Saturday!

On a 700 mile road trip, 100 miles from home, pulling a steep four mile grade in the mountains, what must have been a very small breach in the head gasket let loose. Pressurized the cooling system and blew the coolant out the cap.

Hope to pull the head this week and try to determine the full extent of the damage. Should also be able to determine if it was as I thought, a long term head gasket issue.

Jim

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2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)