whenever i take my gas can off to fill my car up with gas, i always notice a hissing sound when i screw it off. like its under pressure. i didn't really think anything of it until it was brought up by a friend who said the same thing happens to him. he said that he thinks its unusual and is bad for the car, since the fuel pump is having to deal with the pressure in the tank, consequently reducing gas mileage. i was just wondering if he was right or if it is just nothing to worry about. thanks. >>

mine does it too,I am positive it's fine,it says on the cap to remove slowly cause of pressure.

PlanetFuzz is right! That is normal.... you probably notice it usually happens alot more when you are very low on fuel compared to if you had half a tank and just wanted to put in a few $ of gas.
Yep....the reason for the warnig on the cap saying remove slowly...
Just don't be lighting a cig. up right next to the friggin thing when you are removing the cap and you don't have to worry >> HAHA
I hope nobody would ever do that to test my theory but.....
Some people's kids???
HAHA

release of a small amout of pressure

yeah, i didn't think my friend was right, but still had to question ya know. thanks for the help. >>

Your friend his right... but the incidence is not so bad... You don't to be worry about it, I guess

Basically when the car is running, it's a pressurised system - the engine is sucking the petrol from the tank. As it's a rigid construction you get a vacuum due to the small volume of air in the tank with the petrol. The hiss is just the pressure beig released.

This post has been edited by BenMMellor: Jan 28, 2004 - 1:40 PM

it's just your car farting dude!
hahaha