Okay, time for logical troubleshooting-simplest things first.  If the head gasket has failed and there is blow by, the tank will overflow from the pressure, yours is not.  If the rubber gaskets are leaking, that should be obvious.

Same thing happened to me, and both the rear heater core and front heater control valve (above the spare tire) were leaking.  Check those first, and pull the rear heater core to be sure.  Drop the spare to check the front heater valve.  

Don't get a cheap valve, I did and it failed in a month.  $5 from NAPA, plastic is recycled junk and broke, and it's a real PITA to replace. Get the OEM valve from the dealer or one of our vendors.

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Neil N
Sent: Tuesday, April 28, 2015 3:33 PM
To: vanagon@GERRY.VANAGON.COM
Subject: 2.1 WBX Coolant Loss: Searched

Hi all.

After a recent 300 mile trip, noticed coolant bottle was empty,
coolant expansion bottle about 1/3 empty. This may have occurred over
more miles but was a relatively "sudden"
event. I suspect coolant may be getting burned at highway speeds. Details below.

If a head nut isn't obviously loose, should I leave those alone? (not
torque down)
I'm sure I can safely feel if a fastener is obviously under torqued
but does it take a really experienced hand to feel if a head stud is
about to snap?
Is it ok to do a leak down test with engine up to temp?

I recently replaced all cooling system parts except for front-rear
heater cores, hoses heat control valves. A head may have been replaced
by the previous owner. The system bled down correctly after my work,
there were no visible leaks. I currently don't see or smell any signs
of coolant leaks but will check hoses again. I've read that a leak can
be small enough to be an issue over long term but not easily seen.
However. The reason I suspect  coolant being burned is the relatively
sudden change in coolant level.

Thanks

Neil.

--
Neil n

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