Check the small screen, if its getting plugged up you may have some scale or crud in the cooling system that is responsible for it not operating properly.
Has anyone ever seen the pulse that monovalves make at idle with the heat on?
I never understood why it does thet.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!