01 LS430, 107K
WP & TB changed at 90K,
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on freeway, wife spotted temp guage climbing towards H...
I test drive with same symptoms, both fans spinning, radiator full of coolant.
turn on heater on HOT; to help lower coolant temp.
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bought new thermostat from Lexus Concord, they even matched Toyota's price ($4 less) :)
swapped in new gasket & t-stat.
temp back to normal (halfway) :D
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put old t-stat in hot water, heated to boiling.
did NOT open....