[Vwdiesel] thermostats.

[cass]

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hi kids...
so..i was just curious...
why do some thermostats last seemingly forever and some not?
(ones attached to waterpump)
i had just replaced my thermo when i rebuilt the motor last year..
now it is stuck shut.
ive had them work for years,then slowly get old and tired..and not work efficiently anymore,( gawd..sounds like me  :o/   )
but why do they sometimes just get stuck shut.?
not last very long?
seems nothing was obstructing the spring...
car hasn't been run dry or over heated..
nice diesel antifreeze in it.
any wisdom on this ? :o)
thanks
cassie