The Vanagon cooling system can suddenly overheat when there is air in
the system or a slow leak. Having such long water pipes the bubbles
can cause sudden air locks when they get in the wrong place. Either
air gets in the wrong place or the level of coolant drops to the
point that it will suddenly cook and the needle go full over and the
idiot light come on.

I had three sources of leaks in the system that caused this over the
last year in my 84 Westy. They were all hard to find:

(1) The knob on the plastic screw valve of the rear cabin heater
cracked from fatigue and peed fluid very slowly.

(2) The water pump wore out and peed progressively from a location
invisible to inspection in the centre of the engine block.

(3) The plastic air bleed valve in the engine compartment that
connects the two main coolant hoses cracked from fatigue in the same
way as #1. Interestingly this was in turn caused by the 1/2" diameter
flexible rubber hose running from the engine block to that valve.
Over time (19 years), the rubber hardened and it lost its
flexibility, thereby tranferring engine vibrations and torque
reactions to the valve fitting, thus fatiguing it.

Until I found these leaks I had to simply be ready to top-up the
system at the roadside. It was quite a quick routine: unscrew water
filler cap; fill with funnel and 1 gallon can while revving the
engine with the other (right) hand on the engine block throttle lever
to drive bubbles through the circuit; while still revving, screw the
cap back on. if you let the throttle go with the cap off, a whole
bunch of fluid gushes out and you end up with air still in the
system. Got the routine down to abut 120 seconds by the side of the
freeway.

Cheers

Peter