Hey it sounds like you may have a leaking head gasket. The combustion gases
are getting into the cooling jacket under pressure.
Problem!

>I have a 1990 Vanagon that percolates the coolant when I drive to the Summit
>in Los Gatos and turn it off. Ive replaced the thermo fan switch to a  lower
>temp
>and it helped but still percolates. Anybody have a solution?