While washing out the rust in the coolant pipes today, I discovered this valve behind the air filter. What is it supposed to do and should it be open or closed. I zip tied it open.
Mike

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I've not seen one of those on a W123. It looks like a vacuum controlled water control valve, that throttles/controls the amount of hot coolant allowed into the heater core. If it is open the heater core will receive hot coolant and provide heat in the cabin. If it is closed it would block the flow of hot coolant into the heater core. If cleaning the system it must be open. In the summer keep it closed to keep the cabin cooler, in the winter open it to provide heat.

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It feeds water to 3 smaller pipes. Is there 3 heater cores? Just odd. My heater switch is on the other side over the oil filter.