Sounds kike a bad head gasket or cracked head. While the engine is
revving, the combustion gasses are getting into the cooling system. You
do not see the "bubbles" while the engine in being revved due to water
pump pulling through the coolant tank. When the engine returns to idle,
the pump pulls less allowing the air the enter the tank quickly,
releasing the bubbles. This is why both the cooling systems on both the
diesels and water boxers have problems after extended highway runs. The
gasses keep building up until there is no place to go and they either
vapor lock the pump or blow everything out the relief valve.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of stephen de la salle
Sent: Saturday, August 17, 2002 6:51 PM
To: vanagon@GERRY.VANAGON.COM
Subject: bubbles in the coolant overflow tank on 82 diesel westy

Rebuilt engine approx 4K miles ago - for the last 2K miles I get a fair
bit
of dampness and dried salt around the top of the overflow bottle - tunrs
out
hat if I rev the engine, & hold it - no problems. If I then let the revs
drop back to idle - I get a 'burb' of gas coming via the metal expansion
tank cap to the oerflow tank.  It's a puzzle, if it was a leaking head
gasket I'd expect bubbles as I rev the engine - but I only get them when
the
engoine returs to idle....

any thoughts out there....

steve