Pressure build up in the fuel tank!

Hi everyones...thanks to read. My father drives a Chevrolet Astro (1987) and there's a strange pressure build up in the tank. Every two~three days, there's a noticeable odour in
the truck and when the fuel cap is unscrewed, we can hear (and see!) the fumes escaping out, for as long as 3~4 seconds, with a loud hissing sound! The truck belongs to his boss so my father told him about this problem. The mechanics that regularly repairs the cars and trucks for the company told my father that "this happens sometimes"...that he happened to own a Ford that were doing the same thing... and to not worry about that. I personally don't like such answer and i always thought that the pressure inside and outside the tank was supposed to be the same...isn't it? I don't know much about auto mechanics, but does the fuel thank is supposed to be equipped with a vent of some sort? And anyway,how pressure can build up like this? TIA for any replies.
-- Alain(alias:Kilowatt) Montréal Québec PS: 1000 excuses for errors or omissions, i'm a "pure" french canadian! :-) Come to visit me at: http://kilowatt.camarades.com (If replying also by e-mail, remove "no spam" from the adress.)
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The Evaporative Control System isn't working:
the mechanic obviously doesn't know what he's doing or talking about. Tell him to buy: "Evaporative Controls For Dummies" but in the interim. I suggest you find another mechanic.
Refinish King

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Thanks for the reply. I did found and read the following document: http://www.autoshop101.com/forms/h62.pdf so i do now have a better idea of what this system consist of. My father did read the reply and he also said that the fuel tank was replaced two weeks before this problem started. It may be related...maybe not. I'll keep in touch since this truck is supposed to go to the annual inspection in a few days. I'm curious about what they may find when my father will tell them about this problem...
-- Alain(alias:Kilowatt) Montréal Québec PS: 1000 excuses for errors or omissions, i'm a "pure" french canadian! :-) Come to visit me at: http://kilowatt.camarades.com (If replying also by e-mail, remove "no spam" from the adress.)
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It's normal for some pressure to build up in the fuel tank because of the fuel vapors coming off the gas. These vapors are not allowed to escape from the fuel tank because of the hydrocarbon pollution this creates. The vapors are supposed to be stored in the charcoal canister, and when the vehicle is driven these vapors are drawn into the engine and burned during the "canister purge" cycle (how exactly this happens depends on the vehicle). Sounds to me like there's some blockage in the evaporative system, or maybe the canister purge isn't working properly, and allowing too much pressure to build up..
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Robert Hancock Saskatoon, SK, Canada
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