Bypass solenoid valve

Hi,
I just was told that my bypass solenoid valve was bad and needed to be replaced. First of all, what is it and what does it do? The mechanic's accent was so thick that I could barely understand what he
was saying. I know it is part of the EVAP but I am not sure of its function.
Please help!
j.
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Year? Make? Model?
Is this a Honda?
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TeGGeR®

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Don't wanna tell me what year and model? Oh well.
The EVAP system regularly applies vacuum to the vapor purge tank to clean it out. This vacuum must not be allowed to pull new vapors from the gas tank during this operation, so there is a "vacuum cut solenoid valve" to block vacuum from reaching the gas tank. When the EVAP system does its "self test", it needs to be able to apply vacuum to the tank to check for leaks, so it has to override the "vacuum cut solenoid valve". The bypassing is done by the "bypass solenoid valve".
If the bypass was bad, the ECM probably saw the wrong voltage values from other sensors in the system, and it then knew to turn on your Check Engine light with an error code.
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Greetings,
Thank you for the thorough response. By the way, It's a '99 Accord Coupe, 4cyl. I didn't realize that this part would perform different functions in different cars, which is why I neglected to provide the model and year.
How often does the EVAP system do a self test?
When you say, "it needs to be able to apply vacuum to the tank..." Are you talking about the vapor purge tank or gas tank? I would assume the gas tank since it overrides the vacuum valve, which prevents access to it.
Also...the vapor purge tank. How are there "extra" vapors where they need their own tank?
Thanks again!
Jessica
TeGGeR® wrote:

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Performs the same function in all vehicles with that type of EVAP. But...
It's *always* a good idea when posting questions to state year, model, trim level, transmission and presence or absence of A/C. Engine controls differ given the above. This way some of us can look to see if there are known problems, gotchas, or TSBs involving your ride.

Don't know for sure, but I think it's at least once per trip.

Correct.
There aren't any "extra" vapors. All fuel tank vapors end up in the charcoal canister that is called the "purge tank". This is the entire purpose of the EVAP system.
It's just that when the EVAP system attempts to drain the purge tank of its existing vapors, it makes no sense to allow new vapors into the purge tank at the same time. That would be a bit like pulling the plug on your bathroom sink while turning the tap on at the same time.
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Thank you so much for your help! I really appreciate it.
I checked out your website and was fascinated with all the FAQs! I, too, have a buzz at certain RPMs (typically when I'm at a low speed in 2nd or 3rd) in the rear of my car. I wonder if I have the Honda "A-pipe" rattle?
My friend said something about Honda offering a "buzz" kit to fix the problem but I never looked into it because it didn't seem all that problematic.
Thanks again!
Jessica
TeGGeR® wrote:

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to be attempted. Quantity of fuel between X and X; Temperature between X and X and engine cold or the first start in the day. Often when dealing with the evap system you may have a driver that never meets the requirements so the test never gets performed.
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Stephen W. Hansen
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I was wondering about that. Obviously the test would have to be performed under certain conditions, and if those condtions were never met, the test would never be performed. I guess another example would be the cat efficiency test (P0420 error).
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