Pressure build up in fuel tank, 5kcstq

[DeWitt Harrison]

When I open the gas cap, I get a little rush of air indicating there
is positive pressure in the fuel tank. This has been going on
for so long, I can't really remember if it was ever greatly different.
The engine idles and runs well. Now, however, the pressure
is apparently sufficient to push fumes past the gas cap seal
and I get gas odor after stopping the car. No fuel leaks
found after a thorough inspection.

Significant improvement was realized by adding a thin rubber
washer to the gas cap seal but I believe I'm still getting gas
smells from somewhere. This suggests to me that something having
to do with the tank ventilation system is plugged. So far I haven't
been able to locate a blockage. At least a small amount of air
will pass though any hose I blow into including most of the
charcoal canister plumbing. Charcoal canister solenoid
operation also verified. Do charcoal canisters become blocked?

Does this scenario sound familiar to anyone? TIA.

DeWitt Harrison
Boulder, CO
88 5kcstq