[eschaider]

Gasoline uses what is called light ends to enhance octane rating. The light ends in gasoline boil (vaporize) below 98˚F. On a warm day these light ends are easily boiled off in the gas tank. The problem is aggravated with the use of a return style fuel system that has the fuel system pressure regulator located in close proximity to engine compartment heat.

Return style systems will vaporize the light ends in the pressure regulator and return them to the fuel tank as a vapor along with the now low octane base stock the fuel was originally made from. That vapor that is now in the tank is the whoosh you experience when you open the tank. This is a double whammy, as Al Capp's Mammy Yokum liked to say.

The first whammy is the vaporization issue. The second is, like a soda that has been stored too long and lost its 'fizz', the lost fizz can not be put back in the soda. Neither can the light ends be put back into the gasoline base stock! When you open the soda can, the soda is flat and just like the soda so is the gas in your tank. What used to be 93 (or hihger) octane is now down to or near to base stock octane ratings of 88 or 89 octane, sometimes even lower.

The easiest fix for the problem is to move the fuel pressure regulator as far from the engine bay heat as possible. The harder fix is to convert to a returnless system. Detroit has embraced returnless systems for a lot of additional reasons but the boiling off of the light ends and subsequent negative impact on emissions testing / compliance is certainly at the front of the parade.

The warmer the climate the more pronounced the problem becomes, because the fuel tank is also heated by the ambient air — which in AZ can get fairly toasty. The easiest fix is relocating the fuel pressure regulator and parking in the shade (or you garage) when you're not driving it.

The reason that the returnless systems fare better is there is no pressure regulator returning unused fuel to the tank. In fact the fuel still boils and still separates the light ends from the base stock in a returnless system but, and this is a biggie, there is no place for the light ends to go — except into the cylinder which is the destination they were originally intended for.

When the intake valve opens and the injector fires, the vaporized light ends and the gasoline which is itself now being vaporized, in preparation for ignition, mix as vapors and burn in the engine as they were originally intended to do.

FWIW my brand new BMW has the same whoosh performance when opening the gas cap if I am at my AZ home in the summer months. During the winter months — no whoosh. The fuel system is a returnless system so just going returnless is not a panacea. Keeping the fuel pressure regulator and fuel tank cool are the easiest, least expensive and at least at this time, best fixes.