Once, during a trip, when I discovered that my thermostat had failed
open, the engine actually started running rough as it was very cold out.
  My temporary fix was to put a sheet of cardboard (I usually carry some
under the rear cushion for something to lay under the van on) in front
of the radiator.  I covered better than 3/4 of it and, after a time, the
gauge actually started to show normal temperature.

Mike