From the Way Back Machine
There was a post after this reply that wrapping the exhaust with exhaust wrap (aka header wrap) may assist as well.
The answer is basically a combination of the gas tank over heating because it's located directly aft of the engine (most vehicles have the gas tank further away), over filling the tank (which I did the very first time I filled it), and altitude.
"This has happened to me as well, in fact just about every time I drive in
temperatures over 90 degrees it happens. From what I have been able to figure
out, it's a common problem for the Eurovan, as the gas tank is right behind the
engine and all the hot air from the engines flows past and around the tank.
When you reach a certain temperature, the fuel starts to boil and the pressure
inside the tank soars, and if there are any holes/gaps in your fuel lines
(including the one to the generator, which is where I have issues) they're going
to start leaking.
Most vehicles the fuel tank is a long way from the engine air flow, the Eurovan
was almost unique in putting it so close and right in the air flow.
Don't rush into the VW dealer for an emission job. I was quoted $1,200 to
replace the EVAP canister last year. The part was $296 making the labor roughly
$900. A local independent shop doing only German vehicles quoted me the same
$300 for the part but only $400 for labor. I did the job myself in an hour for
just the $300 for the canister. Having done the first one (on my back), I could
do one in 15 to 20 minutes max. It's six screws, 2 bolts and 3 lines.
The place to start is the purge valve. It's a small round barrel looking valve
near the firewall about an inch and a half long and maybe a little larger than a
quarter in diameter with a hose on each end. When the system is working properly
as fuel evaporates in the tank, its accumulated in the EVAP canister, which is
just a big plastic box filled with charcoal pellets. It's stored here until the
engine is started. When the engine is started the purge valve opens and the
engine vacuum draws those fumes from the canister into the intake manifold where
they are burned rather than released into the atmosphere like the good ole days
when tanks were just vented to the outdoors. To test the purge valve, first
locate it. Then start the engine and let it idle. You can touch the valve and if
it's working properly you should feel a slight rattle or vibration as the little
ball moves back and forth. If the ball (valve) is stuck, you might be able to
clean it with a good solvent like mineral spirits, or carb cleaner until you can
get the ball to rattle. If cleaning won't get the job done, a new one is about
$65 from VW if memory serves me correctly.
Here's my caveat. When the EVAP canister fails it's a result of topping off the
tank. The pellets were designed to handle vapor, when you top off the tank
liquid fuel flows into the canister, in effect enlarging your fuel tank capacity
by about a gallon. Unfortunately this turns the pellets into a charcoal mud,
that will eventually outgas the fuel vapors "maybe" allowing a slight venting,
which is likely to sound like the gas boiling in the tank. The next step will
likely end up being only able to get less than a tenth of a gallon into the tank
at a time before the shut-off stops it. It will take you 45 minutes to fill the
tank when that happens. You'll see 18-wheelers filling and going while you're
still there with the hose stuck in your spout. Check the purge valve first (mine
was fine), and if that's not it….look to the canister….sadly there's no test
Basically, if it only happened once, my understanding is the pellets in the EVAP canister should be able to dry out and function again. If you flooded it too many times or got unlucky, you should test that part and replace it if it failed.