I recently had a problem with a sudden overheat condition developing
on my '85 Vanagon.  I noticed a drop in heat output from the front
heater simultaneously with the onset of the problem.
    All the ususal stuff was checked, the thermostat changed, etc. to no
avail.
    The water pump was removed, and the impeller was found to be
somewhat loose on the shaft.  As the engine warmed up, the impeller fit
got looser, and the point was reached where it stopped all pumping
action.
    I spoke to a couple of mechanics who said that they had never seen
this type of failure before (I had the pump in my hand to show them).
    How many of you on the list have had this problem?  The key to
diagnosis is the heater output behavior, I feel, should something like
this happen to you.

Chuck Hill