"hissing" Sound Coming From Fuel Tank

BIF623 · July 30, 2009

I have a 2003 GMC Sierra 2500HD 4wd with the 8.1/Allison. Last weekend I took it camping and towed my pop up tent trailer. After a long and hot trip back home to Phoenix, where it was 110, when I pulled into the driveway I noticed a "hissing" sound coming from underneath the truck. It sounded just like an air leak in the tire, but it wasn't (when shut the truck off it stops) so I crawled underneath while it was running and traced it back to the fuel tank, right smack dab in the middle of it. I've never heard, or at least noticed this before, and the next morning it did not do it. But it was doing it again the next afternoon, although it didn't seem as loud.

I'm paranoid since my fuel pump has been replaced once already. Any idea what it could be? Thanks.

bill190 · July 30, 2009

Try removing your gas tank cap (where you fill it with gas) and see if that makes the hissing stop.

There is an evaporative emissions system which "vacuums" the fumes out of the tank more or less. (as opposed to releasing these fumes to the air and causing air pollution!)

Anyway one of the tubes associated with this might have a leak is my guess. And if you remove your gas cap and allow air to flow into the tank, and the hissing stops, then I would think it would be a good indication it is something to do with this vent system.

If removing the cap does not stop the hissing, then perhaps the sound is coming from the fuel pump?

If an air leaking tube problem, maybe you could peek up there with a mirror and flashlight and see something?

As a rule, these things will be in the most inaccessible place possible!

zackafee · July 31, 2009

Maybe a snake found his new home on top of your gas tank! Haha jk jk

BIF623 · July 31, 2009

Maybe a snake found his new home on top of your gas tank! Haha jk jk

Ok, perhaps "hissing" wasn't the best descriptive word..........

Zembonez · July 31, 2009

Maybe a snake found his new home on top of your gas tank! Haha jk jk

Ok, perhaps "hissing" wasn't the best descriptive word..........

Could be a fuel pump making racket. They are known for that. My 01 Silverado made noise from day one and was still making noise when I sold it at 90K. I'd try the fuel cap like sombody above mentioned. When you hear this remove the cap slowly and see if there is excessive pressure building up.

Alljackedup408 · August 2, 2009

Maybe a snake found his new home on top of your gas tank! Haha jk jk

SubTech · August 8, 2009

I have a 2003 GMC Sierra 2500HD 4wd with the 8.1/Allison. Last weekend I took it camping and towed my pop up tent trailer. After a long and hot trip back home to Phoenix, where it was 110, when I pulled into the driveway I noticed a "hissing" sound coming from underneath the truck. It sounded just like an air leak in the tire, but it wasn't (when shut the truck off it stops) so I crawled underneath while it was running and traced it back to the fuel tank, right smack dab in the middle of it. I've never heard, or at least noticed this before, and the next morning it did not do it. But it was doing it again the next afternoon, although it didn't seem as loud.

I'm paranoid since my fuel pump has been replaced once already. Any idea what it could be? Thanks.

I have an '01 Sierra I had the same sound 'so to speak'. mine was the hard plastic line that comes up from the fuel pump, this is internal to the fuel tank, had vibrated against the one of the springs that holds the fuel pump against the bottom of the tank and had worn a hole in the line, latter on the the truck would not start right up as usual. when you turn the key on the fuel pump runs for a few seconds so that it has pressure to the injectors @ start up. I would have to cycle the key several times in order to build up enough pressure so that it would start without 'cranking a while' when in actuality the fuel pump was trying to supply pressure.

Hope it isn't your pump again this is how I ended up diagnosing the problem with out dropping the tank.