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I have a 2004 Honda Civic EX coupe 1.7. At the end of the fuel rail there is a little plastic tank with an opening with a green cap. I think it has to do with a vapor line. When I swapped engines I noticed the tank on the newer engine did not have the green plastic cap, it was open. So I put a vacuum cap on it so it would not leak. Could someone please explain to me what this little tank is for and should the tank have a schrader valve at the opening for testing purposes. I can't recall if the other tank had a schrader valve. Thanks

 
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This thing? It is a service port for the EVAP system. No idea if it has a valve but I would guess not. Would cause a vacuum leak if left open.

 
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Okay, Thank you, I appreciate the information. That's all I need to know.
Mike