My car is building gas when I open the gas tank it releases the gas, I know all about purge system and I have tried different cap and also different vapor canister too but problem still insists, for some weird reason there is no EVAP sensor in gas tank.

  • Welcome to the site. Can you please clarify what you mean by "it releases the gas"? Does the liquid actually flow out? Or do you mean there is a hissing pressure release when you open the tank? Does this happen all the time or only when the tank is at a certain level (empty/full)? Also, please tell us the make/model and year of the vehicle.
    – CharlieRB
    May 1, 2018 at 15:41
  • @CharlieRB there's the hiss sound and it feels like a bomb has been released because it is so much, and i always have my tank half full, i don't full it because of heat in my country, and it hiss sound and gas release is always there even if i drive my car for 4-5 kilometers, i have checked other cars of this same model and they don't even have a slight hiss sound.
    – ZBT248
    May 1, 2018 at 15:44
  • Perhaps they don’t tighten the fuel cap down tight...
    – Solar Mike
    May 1, 2018 at 16:00
  • @SolarMike they tighten the cap fine, I've checked their caps with my car and still air buildup, and on their car with the same cap there was no gas.
    – ZBT248
    May 1, 2018 at 16:01

1 Answer 1


TL DR: Most likely this is not an issue, but rather the EVAP system at work.

Every emissions controlled vehicle has an EVAP system which controls the amount of fuel which is exhausted to the atmosphere from the fuel system/tank. The EVAP system can test itself one of two ways, either through pressure in the tank or vacuum. Either pressure or vacuum will cause a pressure release sound as you open the fuel cap. If pressure is present, it can escape through a whoosh type sound. If vacuum is present, atmosphere can escape into the fuel tank through a whoosh type sound. It's very hard to distinguish between the two unless you know how your EVAP system works.

If there was an issue with the EVAP system, the system itself will let you know through a check engine light. My suggestion to you is to fill the tank to nearly full (between 3/4 and 7/8 full) and see if things decrease any. I really don't believe you have a real issue.

  • I know however evap system works as I have researched about it a lot, and when there is pressure inside the tank my gas pedal starts to feel heavy i literally know when car have pressure buildup inside it's tank, and no my car don't have evap detecting system, even though it is EFI.
    – ZBT248
    May 1, 2018 at 16:30
  • @davidweb check the vent for obstructions something like a spider web or debris can cause lack of venting.
    – Ben
    May 1, 2018 at 16:46
  • @Ben which vent you are talking about? There are three pipe delivery, return and vapor vent line.
    – ZBT248
    May 1, 2018 at 16:50
  • @davidweb the vapor vent line
    – Ben
    May 1, 2018 at 16:55

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