Doug, I hate to tell you this but you may have a combustion leak into the system.  I have a 2.1 water boxer here that has been doing the very same thing and have determined that the left head has a minute crack in it, so that when driving, over pressures the system and blows stuff up.  At idle all is fine !  So�.. I know that engine is new to you but not NEW, new !  The head could have a slight crack that opens up when driving and that's what is over pressuring the system.  The way I found out what was happening here is by using a test of the coolant for exhaust gases.  John had the kit and we tested the coolant last week and boom there was exhaust gas in the coolant.  Only way that got in there was a crack in the head.  AND, on the water boxer with two heads you can check the temp of the heads.  The left head was reading over 100 C while the right head was at 40 C !  

On 2013-07-29, at 12:00 AM, Automatic digest processor <LISTSERV@GERRY.VANAGON.COM> wrote:

> Re: Checking pressure cap function???

Cheers,    

Frank Condelli
Almonte, Ontario, Canada
'87 VW Westy, '00 Kawasaki 250 Sherpa, "98 Ducati 750 Monster & Lionel Trains (Collection for sale)
Frank Condelli & Associates - Vanagon/Vanagon Westfalia Service in the Ottawa Valley
BusFusion a VW Camper camping event, Almonte, ON, June 06 ~ 09, 2013