Volvo Repair: Volvo 850 - Petrol Smell, type of hydrocarbon, fuel vapors

Question
I drive a 2.5L, 1996 850 StationWagon (T5). I just got the car a few weeks ago. After any trip, I noticed a petrol smell everytime I step out of the car (Right-Hand Drive) - the same side as the Petrol Tank lid. The specialist told me that it may be due to the petrol tank's breather hole (?). If it has a breather hole, wouldn't it mean that given the very hot weather nowadays, the petrol would be evaporated? I wasn't convinced by this reasoning. And further, I noticed that the fuel consumption isn't as displayed by the electronic display. The electronic display shows 15.7L/100Km but the fuel gauge shows that slightly more than 20 litres has been consumed. Adoi! Which is the the more accurate measure?

Thanks for your advice.  

Answer
The tank does have a breather hole.  If it didn't, then when you were driving and using the gas up, the level would go down but no air would come in to fill the space and you'd pull a vacuum in the tank.  That can't work so there is a breather hole; It doesn't evaporate away though because there is a charcoal canister inbetween the tank and the breather hole.  Charcoal has a special charactaristic that it holds any type of hydrocarbon, including gasoline.  Normally thats how it works but if your canister has become damaged, or the purging of it isn't occuring correctly then you may have some fuel vapors escaping into the air - hence the gasoline smell.  The fuel consumption measuring is interesting.  The display calculates fuel consumption based on fuel pressure and the amount of injection time.  It is usually very precise.  The gauge has an older float system that can tell how high the level is and give a general area of where it is.  It isn't off by very much though unless the tank is totally full or empty.  You could probably account the difference(in your case) to a fuel leak.  The display is showing what it used, and the gauge is showing what you have .  So take 20 litres minus 15.7 and you get 4.3.  A rough number of what you are LOOSING per 100km due to a leak.  Best get it fixed.  If the shop suspects a fault in the vent, then let them fix it.  If it doesn't fix the problem when the ysaid it should have then they're obligated to fix it again, at no charge to you.  Hope that helps.

James