Hissing noise when opening the gas cap

[Nas Escobar]

So I've been slowly working on the 93 Z34 and one of my concerns is whenever I open the gas cap to fill up, it hisses. I guess "vents itself" would be the correct wording, but what is the cause of it and should I be concerned about it in an Emissions state?

[Imp558]

That's normal, mine does it sometimes too. The gas tank is under vacuum when the car is running, that's what a FTP sensor is for. Newer cars will pop a code if there's no vac in the tank.

For whatever reason it's more likely to happen when there's little gas in there which is why we would take the cap off in the first place.

[digitaloutsider]

Agreed, it's normal.  My 2nd gens never do it, but all of my 1Gs did, especially my Z34.

[Imp558]

In this region Country Fair stores do 5 or 10 cents off with a gift card purchase so I rarely hear the hiss now. By the time I buy the card and come back out there's no residual vacuum in the tank.

[jman093]

For whatever reason it's more likely to happen when there's little gas in there.

 

That's because less gas means a larger volume is under vacuum in the tank.

[RobertISaar]

GM's statement on it is that the relatively long threaded portion of the cap will allow any existing pressure or vacuum to vent before it is completely removed, therefore not a problem?

 

I would think the carbon canister would be sufficient to relieve any building pressure or vacuum? there is supposed to be a restrictor between the tank and the canister's feed port, which would slow the venting process, but it isn't like the canister would have to suddenly equalize pressure as soon as the engine is off.

[Imp558]

I wonder if that has a solenoid valve to the carbon canister? That could have failed, but then again they all could have failed because mine does it too

[94 olds vert]

This is the first I am hearing of this. My 'vert has never done this. I guess I won't act surprise when it does. 

[Nas Escobar]

I wonder if that has a solenoid valve to the carbon canister? That could have failed, but then again they all could have failed because mine does it too

I've had 3 W bodies and only the Z34 hisses. I've ran both my Buick and my Cutlass with constant 3 gallons in the tank and filled up with 1 gallon in the tank. None of em ever hissed. I usually keep the Cutlass with 4 gallons to start it and move it around and I ran it dry and it didn't hiss. I opened the tank with 2 gallons in it and it hissed. Where would the carbon canister be? How would I check it?

[Grandprix1]

I've had 3 W bodies and only the Z34 hisses. I've ran both my Buick and my Cutlass with constant 3 gallons in the tank and filled up with 1 gallon in the tank. None of em ever hissed. I usually keep the Cutlass with 4 gallons to start it and move it around and I ran it dry and it didn't hiss. I opened the tank with 2 gallons in it and it hissed. Where would the carbon canister be? How would I check it?

I'm not 100% sure but look back around the back wheel wells. I think that's a pretty common place for them. Should be a smallish black box with a line running from the gas tank.

[movielover40]

My wife had a Ford Probe that did it all the time.

 

Nothing to worry about.

[RobertISaar]

i'm wondering if some of us are dealing with slightly defective or purposefully "sticky" vacuum release valves in our fuel caps.... the cap has to vent vacuum as fuel is used to prevent the pump from having to draw against a vacuum, I'm thinking the release threshold of the caps are probably different. try moving the cap of a car that hisses to one that doesn't and see if the noise follows?