hear air being sucked into fuel tank???

Hi Guys
`95 Firebird ,3.4 .......110,000 miles. When I twist fuel cap off I can hear air being sucked into fuel tank. Not likely a good thing . No SES lights yet . It should vent...........correct. Haven`t had the car long , seems a little shakey upon start up .I know I have to do plugs and likely wires. Fuel filter has been changed . Main concern right now is the non venting issue
Thanks Muff

Yeah, your fuel tank is just completely airtight. Dont think its a big deal, but I'm no expert.

It's normal, both my cars do that.

You mean you hear air when you take the gas cap off to fill it up?

Mine does the same thing, I just assumed it was normal lol.

ok thanks for the input. I know my `00 Silvy doesn`t do it,just thought it was odd, Thanks Again

The fuel tank is vented through the gas cap and EVAP system. The fix could be as easy as chnaging the gas cap.

The fuel tank is vented through the gas cap and EVAP system. The fix could be as easy as chnaging the gas cap.lol..... so now what you`re saying is that there should not be any vacuum coming from tank when I twist the cap off to fuel . I cannot remember ever hearing this on any of my other vehicles and it does seem odd. The cap looks sealed I was inclined to think there was sometihng going on with evap system. What I was thinking is that too much vacuum will lead to poor performance. Put too much vauum on anything and it will collapse. Not saying it will collapse but seems odd.......Thanks

You have it right. The gas cap is supposed to let air into the tank. Like you said, when the gas is used from the tank, it must be replaced with air. I don't think you could get enough vacuum to collapse a gas tank. But, I do think you could end up burning up a fuel pump faster. The pump would have to overcome the negative pressure in the tank to suck it in. Gas caps are cheap.

I have a few cars that do this but it's usually when the tank is almost empty. I can't see it being anything to be concerned about though. It would make more sense though if it wasn't sucking air in but instead blowing it out ... since an empty tank would have more fumes and build air pressure... the vent system probably is a slow vent and allows the tank to still be somewhat pressurized... a small amount of pressure in the tank would be benificial to the pump.

Under that theory though, if it was fumes and such causing the pressure, wouldn't you get a strong gas smell as the pressure is released?

Yeah, I believe it is air sucking into the tank but I was just stating that the other theory would make more sense. Now that I think of it though, The tank is designed to be under comression because the newer cars will throw a code if the gas cap isn't on tight enough. That could only mean that it isn't designed to be the same pressure in the tank as the outside air pressure.