While my car is idling, I noticed a loud "hissing" sound coming from the gas cap. You can't hear it inside the cabin, but just happened to be outside of my car while it was on, and heard it. At the same time, I have an emissions malfunction indicator light on, and my gas mileage has been poor as of late. So possibly there is a problem with the gas tank pressurizing because of pressure escaping through the cap? Happened to anybody before? Is it the cap, or a bigger problem? Thanks!

 

No, you probably need a new gas cap.

 

Make sure the cap is tight.

 

I mean the cap is getting as tight as it's going to get, i turn it until in clicks.

 

do you have a vag-com so you can check the vapor return line solenoid or you can just feel it while the car is running and hot it will be clicking

have you changed and vacuum lines under you manifold lately it one of the check valves is in backwards the vapor will not be able to get to the manifold and pressure will build in you gas tank...

the cap might just be faulty