Today I took my car to get emission tested. When they tested the gas cap it failed which was not a surprise to me so i got a new one. When i got home I changed the O2 sensor because the car failed, and I heard a buzzing noise. It turned out that it was coming from a valve in the purge line coming from the fuel tank. I took the gas cap off and there was tons of pressure in there. I was just wondering if this is normal. I knew there should be some pressure but this seemed very excessive. I dont know if maybe that valve isn't purging like its supposed to or if this is normal.

 

Sounds almost normal. Your old gas cap failed emissions because it didn't seal properly. Now the new cap is sealing.

Mine hisses for a good 15 seconds when I take it off to fill up. Sometimes it doesn't hiss at all. Something to do with expansion and contraction and gas vapor.

However, I've never heard a buzzing sound from the fuel tank area.

 

Few folks understand just how much gasoline expands and contracts. I manage a gas station - when I order gas, I order 9000 gallons at a time (a full tanker). In the winter, when the gas is warmer in the ground than it would be above ground, they fill the tanker to 9000 gallons - it will contract to 8600 gallons by the time the tanker gets to the store (45 minutes - 1 hour). That's alot of contraction very quickly! Then, when it's loaded into my underground tanks (where it's warmer than above ground), it will expand back to 9000 gallons. It's very cool. They record the load at the terminal, again on the truck, and it gets recorded again after the drop at the store. The truck recording is always less than what the store recording is (in the winter). The complete opposite is true in the summer.

The sad part is, in this technology society, they can now account for this with computerized models - thus I actually pay for the 9000 gallons, even if on the paperwork the tanker only legally dropped off 8600 gallons.

 

haha


Yea, you can see on the gas pumps too. most of them have a sticker stating volume is corrected to a certain temp ( 0*C maybe? cant remember..)