<< Mike if there isn't any combustion gasses in the coolant then where is the
 pressure coming from?  Have you checked for a stuck thermostat? >>

Hey Volks,

    If this person has an automatic transmission....it could be a pinhole
leak in the ATF heat exchanger causing this.... if he has the long-life
orange coolant, you might not even see a color change.  A check of the
overflow tank would reveal some film on the insides of the tank....just a
thought.

Scott De Santi
86 Vanagon 2WD