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Part of the vapor recovery system design (long) Posted by Gene N [Email] ![]() ![]() ![]() In Reply to: But Why ?, Apolo, Fri, 28 Feb 2003 10:32:08 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The charcoal canister up front is connected to the top of the of the gas tank via a long plastic tube. When the gas cap is on the system is sealed. Manifold vacuum (through a check valve to prevent the boost from pressurizing the tank) pulls the vapor out of the canister and builds up a pressure differential in the tank. I've never measured it, but I would guess the tank is never more than a few psi less than the outside air pressure. There is a small door over the filler hole to help keep vapors from being released until the filler hose is inserted. The slow blow by of outside air past this door is responsible for the 5 second whistle you hear. You might notice that the noise stops as soon as you insert the hose, opening the door.
This vapor recovery system has been a feature of most cars since the 1980's (maybe earlier) and most cars exhibit this behavior. Lately, car makers have been moving the charcoal canister closer to the gas tank to meet more stringent emission standares.
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