The black unit is the vaccum control valve that blocks coolant flow when the heating controls is in Air conditioninig mode. The stock Vacuum heater valve has a small bleed hole that allows a small amount of coolant to flow so that the system can bleed air. (The engine does not require coolant flow as some have said) Unfortunatly, this allows the heater core to remain hot, adding heat to the HVAC box.
I replaced the stock one with one that closes completly.
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