RE: Possible Plugged Radiator and Back Flushing
I dealt with (for several years) a bad radiator in an '85 (1.9L engine,
somewhat different coolant system) and in the process of trying to figure
out the problem was told by an experienced VW mechanic that because of the
setup on the Vanagon radiator back fluching and 'flow testing' won't work,
won't help, won't tell you anything.  I'm not sure how true that is but in
my case the radiator appeared to be fine but in fact was clogged and the
back flushing attempt actually made things worse.
A search of the archives will show that there have been many reports of
cooling system problems being solved with a new radiator.  And not that
expensive compared to a cooked engine.