They can weep a small amount for a long time, and the valves can leak too. If you remove the heater core your helper can see evidence of any leaks. Very common failure in Vanagons, and yours is probably in the early stages. Typically, the end cap sealing compound hardens and fails.  At least that's what happened to the one on my '85.

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Eric Caron
Sent: Sunday, November 23, 2014 7:07 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Smelling coolant suspect rear heater

        Hi folks,

        I’ve been out with the van a few times in November and have turned the rear heater valve back to full and have been using the rear heat.  I smell coolant when I turn on the rear fan.  I don’t smell it when the fan is off.  I have not found any coolant on the floor under the seat.  i do have the drain plugs open as a prevention in case of leak.
has anyone noticed this smell before a rear heater failure?  could it be the fan rather then the heater.  When I purchased the van the rear heater fan did not work but replacing the fuse has fixed the problem for over a year.

coolant level is unchanged.

Experiences,and recommendations appreciated.  I’m considering just calling a vendor to change out the core but it seems silly to do if it has nothing to do with the heater core.

Eric Caron 
85 GL Auto