Течь уплотнения заливной горловины

The seal between the filler neck and the fuel tank, or the neck itself, no longer sealing. It matters because the evaporative system is a sealed volume: the vehicle pressurises or evacuates it to test for leaks, and anything that does not hold counts as one.

It is the second thing to check after the cap and before anything expensive, since both sit at the same end of the system and a leak here presents identically to a cap fault.

Corrosion is the usual cause on an older car. The neck sits behind the filler door where road salt and water collect, and rust perforates it from the outside in — often invisible until the trim or the wheel arch liner is removed. Capless filling systems add their own failure: the sprung flap assembly that replaces the cap has seals of its own, and they harden.

A smoke test is the right tool, because a neck leak is frequently too small to see and sits behind trim. Ask for one by name rather than paying for a visual search.

Note that misfuelling protection devices fitted in the neck can also be the culprit where one has been damaged by a badly aimed nozzle.