Well, I don't think it'll just snap back into the liquid phase in one go. It's a while since I last did any thermodynamics, but isn't it the case that boiling the petrol by lowering the pressure will (as for any liquid e.g. CO2 fire extinguisher) cause a drop in temperature to below ambient, so heat will then flow into the fuel line and vapour from the surroundings, resulting in vapour that's not in equilibrium with the liquid phase, and so even when the pressure returns to normal (which heats the vapour even more) it needs to cool down again before it recondenses?

Peter