Greg, the Tempil° sticks I have "Melt" at the temperature shown on the
stick,they do not change color. Send me your snail mail address and I'll
send you a couple.. I also have some "THERMOMELT" sticks which work just
as good as the Tempil° Corp sticks.
It's true that the radiator header tank does not reach the same
temperature as the engine coolant until the thermostat has opened and
flowing coolant for a couple of minutes but I do not check coolant
temperatures until the engine has fully stabilized. If the thermostat
"fails" It would be caught when you made a second Tempil stick check on
the thermostat housing base (Say 10 or 20° above the thermostat valve)
THis assuming the temperature sending unit was located in the radiator
header tank and not the engine block.
If the cooling system is working properly, I feel, you should get a very
small temperature differential between the engine coolant and the header
tank but ,of course, if the temperature goes above the thermostat - Look
elsewhere.
Jim
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