Back when I had my '75 Scirocco, I re-packed the
charcoal canister with fish-filter charcoal.  I was
able to open it's canister, and empty out the gassy
coal and refill with fresh.

Someone mentioned shaking your canister, this may not
show anything since (on the early ones at least) the
intake side filter is held against the coal with a
spring to keep it packed tight.  The new coal comes
kinda wet, I'd air it out first.

Some cars use hot air from the exhaust to help purge
the canister.  I think it's just a good way to store
tank vent fumes, so the benefits may be minimal.

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