does anyone still hear hissing sound when gas cap is removed?

[3100]

Now when I think back several years, when I was able to hear hissing sound coming out of the gas tank when opening gas cap my MPG was much better. Now when I don't hear it I am thinking that gas is evaporating somewhere or
EVAP system is sucking more than needed.

here are some numbers. I was averaging 246 mpg city/highway 50% 50% from completely full to low fuel light, now for the same I get about 200


I once tested regular 2 dl full glass of 87 octane, left if outside over night and in the morning only 1/4 was left.

[T K BHAT]

I have a similar experience i.e. higher mileage when a hissing sound was heard on opening the gas cap.

It is not making that sound anymore and the mileage is reduced by about 20-22%. The service stn do not have any view on this. I am looking into it myself and post result after hissing is restored.

[ricebike]

yes i still hear the sound when i take off the gas cap to fill up... it's supposed to be almost closed system, so we won't vent fuel vapors to the atmosphere...

why there's soo many vehicles with obd II systems that light up the check engine light when the cap isn't properly tightened

[3100]

Originally Posted by ricebike

yes i still hear the sound when i take off the gas cap to fill up... it's supposed to be almost closed system, so we won't vent fuel vapors to the atmosphere...

why there's soo many vehicles with obd II systems that light up the check engine light when the cap isn't properly tightened

What year and engine size is your GA, also how many miles and what is your average mpg?

[ricebike]

this isn't grand am specific... it does this to various vehicles i've driven with the exception of a dodge/freightliner sprinter van (that's diesel-powered)

[andretti]

If there was an EVAP system leak, it would set a service cod and turn on the MIL.