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Black smut

I have black smut forming around gas cap

The seal on the gas cap may be breaking down. Replacing it is cheap.

Thought about problem with gas cap, but when I remove gas cap it makes a sound like the gas tank is under a vacuum. Could it be breaking down if I hear noise?

Thanks

The black smut is coming off the black rubber seal for the fill neck to the gas tank.

Take some silicone spray lubricant and soak a paper towel and wipe the seal down to remove the smut and to prevent it from causing more.

Tester

Is your car getting hard to fill rapidly with gas? If it is then the hissing sound could be from a bad vent on your gas tank. If you can fill it normally then it is nothing to worry about.