I had a similar experience minus the coolant covering the back of the van.
Turned out to be a leaking compression head gasket that allowed the engine
compression to pressurize the coolant.  With high pressure in the cooling system
the radiator cap opened to allow "overflow" to the overflow tank which
overfilled this tank after driving for awhile.  Basically, high pressure air was
displacing coolant.  This only occurred at highway speeds until it progressively
got worse and did the same thing around town.  Had to do the head gasket
replacement.
Mark

Todd Last wrote:

> I was taking my '88 Westfalia down the freeway when, after having to reduce
> spped suddenly due to traffic (from about 75mph), The red water temp light
> started flashing. Then, as I was pulling over, the temp gauge started
> rising.
>
> When I stopped I noticed that the back window was sprinkled with coolant and
> the licence plate looked like someone had hit it with a water balloon. In
> looking at the engine compartment, the expansion tank was very low, but the
> overflow tank (behind the license plate) was full past the top of the fill
> cap.
>
> I filled up the expansion tank and drove on everything now reading normal.
> About 60 miles later the coolant light started flashing - but the temp
> showed normal. I stopped and filled the expansion tank again, and this time
> let the extra coolant drain out of the overflow tank.
>
> I originally thought that there might be a leak in one of the tanks or a
> hose somewhere - but I do not understand why it seems to have fixed itself.
> Could it be that I just had a huge air bubble in the system?
>
> Anyone have ideas?
>
> Thanks
> Todd
> '88 Westy