It appears the intended DesignLifeTime for this vehicle, assembled in 2000, was engineered for around 18yrs; starting in 2019, during nearly each Coolant system maintenance routine (ThermoStatValve, Hoses, FlushFill...) yet another original plastic part (ThermoStatHousing, HoseTeeFitting, RadiatorSideTank...) cracked & leaked. In 2020July, after installing new hoses In+Out of the HeaterValve, it started leaking. Like the ThermoStatHousing & Radiator, I replaced this leaky plastic valve with the AllMetal type below, common on 1970s+1980s vehicles.

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Thanks go to the brave souls who've preceded me in similar adventures & freely shared their experience below: Coolant HeaterValve Fix Coolant HeaterValve Fix Coolant Leak Fix: K-Seal
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