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I had similar symptoms when the thermostat in my Celica failed in the
open position. It was intermittently faulty for about a month and then
failed one winter night. When I got home the fast-idle was on!
The wax capsule was still OK but the valve (which had a rubber seat) had
partially fatigued through and had bent at an angle, so it couldn't
close.
It had a '93 date code on it and was obviously the original.
--
Neil J. Harris