I don't think idling will burp the cooling system, and you may still have an air pocket. What was the new t-stat rated for temp. wise?

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1985 300D-189k The 'UD', Ivory and Pinkamino
1979 300D-211k Dark Gray, Parchment
A 1980 Harley-~166k and
A 1994 Ford diesel pickup-349k and
A 1990 gasser Volvo wagon-145k