When the engine is not running, or at idle, the tank should be sealed.
Fumes are collected in the carbon canister but should not be released to
the atmosphere. When the engine is off idle, the valve for the carbon
canister should open and the fumes vented into the air filter and burned
off in the engine. As fuel is sucked down, replacement air should also be
drawn in through this valve. You should be able to hear the valve click
open when the throttle is opened. One of the problems with most engine
conversions is that the throttles are not ported to properly operate this
valve.

Personally, I do not think it is unreasonable for the fuel system to
remain tight and prevent evaporating fuel from just venting into the air.
By 1971 almost all cars had some type of vapor recovery system. If this
test finds leaky systems that need to be fixed, then it is serving the
intended purpose even if the program is not perfect.

When the tank is filled, it not vented through the carbon canister. The
actual vent is through the filler and the vapor recovery nozzles are
supposed to collect those fumes and bring them back to the stations tank.

Dennis.

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
John Bange
Sent: Monday, March 03, 2008 8:54 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Calif 2008 emissions fuel tank pressure testing?

> Maybe I'm operating under the wrong misconception, but I don't
understand
> how a tank can be expected to hold pressure when it presumably vents
thru
> the charcoal canister to let the air out while being filled.  Can
someone
> explain?
>

Yeah, you got me.  The guy at the test place didn't explain it very
thoroughly, plus I wasn't listening very well. All I know is it involves
pressurization (vacuum?) of some sort, and takes 15-20 minutes to test.
Perhaps a charcoal evap filter only should be able to pass a certain
volume
of air in a given time, and if too much can pass, that indicates a leak?

--
John Bange
'90 Vanagon - "Geldsauger"