It sounds like your headgasket has blown and is letting combustion gases into
the coolant.  I recent found this on a customer's 81 rabbit.  Basically you
could sit and watch the bubbles coming up into the expansion tank when
allowing the car to go from cold to warm with a full coolant tank (should be
able to do something similar on your van).  If it keeps bubbling and
bubbling, then the gasket is toast.
Side question, how does your '87 Vanagon happen to be a diesel?  Non-US?

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242