My guess about the whoosh is air being sucked in. It’s very unlikely that evaporating fuel will lead to a pressure build up to create the whoosh out. As fuel is used by the engine the tank empties creating a partial pressure drop. From my distant physics/chemistry past ( gas laws and related partial pressure/ latent heat effects) I doubt the fuel fumes could lead to a pressure increase but, at best, partly reduce the pressure drop caused by fuel removal. Just remembered...place water in a vacuum chamber, pump the air out. The water boils as the air pressure drops. This demonstrates how fuel can evaporate a bit as a partial vacuum is created in the tank by fuel being pumped out. The pressure will, at best, only reach atmospheric and not go higher to result in an outward whoosh.
Many tanks have additional vents that include roll-over sealing mechanisms. These can seize up to prevent venting inwards to balance the fuel being used, hence the whoosh.
Check for such a vent and function.
Last edited by sospan; 28/08/22 12:48 PM.
