Date:         Wed, 8 Jan 1997 19:47:29 -0800
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         mholser@Adobe.COM (Malcolm Holser)
Subject:      Re: Re(2): Nervous about heads

The caps are easy to test -- they make good duck calls when they work.
Take a new cap, and blow into the little tube:  Great Quackers!  DUCK, Batman!

Seriously, this will test one aspect of a properly working cap (and sounds
real funny at the parts counter of your local VW dealer!).

Sadly, it does not test the second, and perhaps more important desired
characteristic, the pressure at which it allows flow the ther direction.

The cooling system operates under about 12 lbs. pressure.  The coolant
runs about 210-220 degrees (F  -- a little over 100c) and would boil if not
for the antifreeze and (more importantly) the pressure.  This cap regulates
the cooling system pressure.  As the coolant gets hot and expands, it is
forced through the cap, and into the overflow tank when it exceed the
desired pressure.  Your dealer has a tester to measure this pressure.

If there are any cracks or leaks in the system, the pressure is not reached
and the coolant just leaks out other places.  On a sealed and correctly
operating system, when the engine cools, and the coolant contracts, the 
pressure becomes negative (quickly) and coolant is drawn into the system
from the overflow tank.

What goes wrong:
Large air bubbles in the system act as a balance, collecting the pressure
by compressing, and then expanding when the system cools.  This prevents
enough vacuum from forming to draw the coolant in against the "duck call"
pressure of the check valve.  With no coolant being drawn in, the bubble is
not ever purged and the system is in the start of a failure mode.  Small
bubbles eventually migrate to the top of the coolant tank where they will
get purged next time the car warms up.

Leaks in the system cause (initially) coolant to not be expanded into the
tank (it leaks out somewhere) but still allow coolant to be drawn back in,
so you have to keep adding some to the overflow tank.  If the leak becomes
bad enough that enough vacuum is not produced during the cool-down to
overcome the check valve, then the coolant checks outs, but don't check in.
You get the situation where the overflow tank is full when you blow up
your engine 'cause there ain't no coolant where it needs it.  Air bubbles
fail to be purged (since there is not enough pressure at the cap to burp
them out where they collect) leading to the additional problem of the first
case.

A leak in the combustion chamber will cause exhuast to be added to your
system.  This sets up both of these bad cases -- big "air" bubbles and
failure to hold pressure, and probably failure to get vacuum to suck in some
replacement coolant.

On a good, new, sealed, leak-free system, you only need to occasionally
check the overflow tank.  On Vanagons you need to check the REAL tank.
That's where you'll see the trouble.  If you have been adding coolant
in ever-increasing little bits, and then it stops needing any -- this 
is not a sign of health!  This is a sign of you making your mechanic's
BMW payment sometime soon.

The system will actually tolerate quite a bit of small leaks and small
air bubbles -- the '86 and up is better at this, with lots more air-bubble
collecting pipes put there to purge the small bubbles.  They don't do
the big leaks/combustion leaks/big bubbles well at all.  In the bad old
days, when 15,000mile tire warranties were rare, the big iron cars just
put huge radiators in, and you filled-em up.  Often.  The little overflow
tanks helped quite a bit, especially when people routinely started putting
expensive things like antifreeze into their cars, and keeping them more
than 50,000 miles.  The late Vanagons have one of the most elaborate
cooling systems around.

Time has proven that this just adds more points of failure, though.  To
be fair, VW's real problem on the Vanagon was trying to make a Volkswagen
maintenence-free.  Instead, they, like most auto manufacturers, just 
get rid of the small day-to-day annoyances (like checking your radiator)
in exchange for some rather big ugly things farther down the pike.

malcolm

Did you ever notice the similarity between "Pike" and "Picard", the first
and last Captians-of-the-Starship-Enterprise?