On Wed, 15 Aug 2012 19:03:29 -0700 (PDT), wrote:

They cannot be designed to fail open.

J
That is an interesting point. I clearly recall thermostats that
either failed to close completely or opened at too low a temperature.
On the other hand, the usual thermostat design utilizes a wax pellet
that forces the valve open. I can't see any way to design that to
fail open.

Still, that is clearly not the problem in this case. As described,
the engine runs at near normal temperature while the van is in motion,
but overheats when stopped. I have seen that problem once - my
Father-in-law had disconnected the electric radiator fan on his Audi
because he didn't like the fact it ran after the engine was turned
off. The engine would stay at normal temperatures while it was in
motion, but overheat when standing - for example at a traffic light.

PlainBill