I'm pretty sure they're talking about the canister air filter. If it gets
clogged, your tank won't be able to properly vent fuel vapors through the
charcoal canister. When this occurs, you may have difficulty filling the
tank, or the check engine lamp may come on, or both. I've only seen
three cars that had canister filter failures. I think the common theme
driving in dirty and dusty areas.
That may be correct for the Sonata. There was a thread on one of the
message boards about insulating the fuel filler tube to minimize the
sloshing noise. Apparently getting at that tube involves removing the
left-rear wheel well liner. It was mentioned that removing that liner
also gives access to the mysterious fuel tank air filter. ymmv.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.