For what it's worth, I'll report that I now believe the bubbles were from trapped air in the path from the head back through the heater core, past the voltage-controlled water valve, and through the pipe near the exhaust back to the water pump, finally being purged.

I still can't understand how air could remain in this heater-core-related area for so long with routine driving. Is the heater core higher than any part of the water passage in the OM617 cylinder head? I don't think the MB engineers would have designed such a situation.

The car now has fresh antifreeze, distilled water, a new thermostat, and the system purged of air with repetitive blips of the throttle to bring revs up to about 1800. The water pump undoubtedly sent pressure pulses into the head that then exited through the fitting and hose behind the oil filter, moving the air along until only water remained in the system.

I still cannot believe the above but there seems no other explanation since the engine runs smoothly and power is good....... so apparently no leaking head gasket exists.

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Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951