On Tue, Sep 20, 2011 at 10:55 PM, Travis Newhouse
<travisnewhouse@gmail.com>wrote:

>
> Loren: Regarding clogged radiator, I can understand how that would
> diminish cooling capacity, and perhaps only lots of wind at high speed
> is enough to keep the engine cool with a clogged radiator.  But, how
> does a clogged radiator cause bubbles in the expansion tank?
>

A clogged radiator, short of total blockage, may provide just enough
cooling  under light to moderate loads and vehicle moving and pushing air
through the radiator.  But under less than ideal conditions, like  stopped
in traffic or climbing mountain passes etc.  heat builds to the point of
overheating.  Or when engine is shut off and coolant not being circulated
the already to hot engine will boil the coolant, thus the bubbles and
overflow.  At least that is the way my '85 behaved, like the radiator just
couldn't handle the task of cooling.  I had to run mountain passes with the
heaters wide open to get a little additional cooling.  And then have coolant
puking from the overflow tank when I stopped.
BTW, later I was told by at least two experienced Vanagon mechanics that the
stock radiator can't be flow tested properly because of the internal
geometry.  And the end of my saga was a sincere (but not VW experienced)
mechanic, trying to solve the problem, tried to back flush the radiator and
that rearranged the junk that was clogging it to the point I was getting
virtually no cooling at all.  That (and my   pigheadedness)  cost me the
engine.