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Re: Awesome input, guys. Thanks. Posted by sam96CS [Email] (#852) [Profile/Gallery] (more from sam96CS) on Fri, 3 Jan 2014 17:53:37 In Reply to: Re: Awesome input, guys. Thanks., T. Christopher, Fri, 3 Jan 2014 09:07:29 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The fuel cap is non-vented, not one-way vented. If it was one-way vented to release pressure then it would have to be designed to release air without also releasing fuel vapor. If the cap was one-way vented to relieve vacuum in the tank, then tanks would not be deformed by vacuum pressure when the line from the evap canister is clogged or the rollover valve is clogged. The fuel system "breathes" through the charcoal canister, not through the gas cap.
In the old days it "breathed" through a vented gas cap, but not now.
The EPA regards evaporated fuel as just as harmful as combustion emissions. Every manufacturer is required to keep fuel and fuel vapor from escaping, and they are required to monitor the operation of the system that does the job. Drivers of '96 and later 9000s with OBD2 get a P0441, P0443, P0444 or P0445 code to alert them to a problem with the evap system.
When the pressure builds up due to thermal expansion the evap system's charcoal canister uses activated charcoal to separate fuel vapors from air so that air is released but fuel is trapped in the charcoal. The purge valve later releases the trapped fuel into the intake manifold as commanded by the engine's computer, and the computer makes sure that the purging operation does not result in an excessively rich or lean condition.
When there is a vacuum condition in the fuel tank outside air enters not through the gas cap but through the charcoal canister. On the 9000 I don't know whether there is a valve that is commanded open or a passive one-way valve.
The 9000 gas tank "breathes" through the charcoal canister, but air mixed with fuel does not escape.
This is an oversimplified explanation. I don't have detailed knowledge of how activated charcoal works or how the engine's computer manages and monitors the evap system. To me it is mostly magic. I stop pumping when the nozzle clicks off. I turn the gas cap until it clicks at least 3 times. I check the condition of the little rubber hose in the passenger wheel well when I rotate tires.
->Posting last edited on Fri, 3 Jan 2014 17:54:32.
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