Newbie Question: Pressure in Fuel Tank

[NickBarrFishing.com]

I have another simple question, when I open my gas cap, I get a hissing sound, so I know there is a buildup of vacuum in the tank. How much is too much? Do I need to get a ventilating gas cap? Lift Pump precautions??

Thanks,
Nick

 

[Turbine Doc]

OEM cap for Diesel truck will properly regulate the venting of the tank

 

[Pruittx2]

yep camps go bad,, but when you get a new one Make sure its is a DIESEL cap. I just screw mine on till it seats, then back it off a tad, Cause my Autozone seems to have a hard time understanding what a Diesel cap is. They keep getting me a gasser one!! DOH!

 

[GM Guy]

We got a genuine GM diesel cap on the 92, it even has a D on it, and it still allows some vaccum, there is a whoosh when I open it.

 

[NVW]

GMCTD had a thread on the fuel caps and how the vacuum prevented foaming

 

[3500GMC]

Would the foaming be a result of return fuel to the tank?

Any GM part numbers for the caps? Probably different numbers for year, model though.

 

[Chevylover]

Would the foaming be a result of return fuel to the tank?

Any GM part numbers for the caps? Probably different numbers for year, model though.

Not the GM number but have a look here at lmctrock.com :

Cu,
Sven

 

[treegump]

I don't think I've ever noticed a hissing sound when I unscrew my filler cap

 

[Woody35]

I don't think I've ever noticed a hissing sound when I unscrew my filler cap

thats good

 

[bigchevy89]

so is there suppose to be a hissing sound when you unscrew the cap?sometimes there is a lot of vacume in my tank.

 

[Jess_sr]

For what it's worth I have vacuum in both mine and my sons truck.
I see no problems with fuel starvation.

 

[Woody35]

so is there suppose to be a hissing sound when you unscrew the cap?sometimes there is a lot of vacume in my tank.

you dont want hissing

 

[GM Guy]

well then why does our all new setup on the 92 still hiss? The filler neck was replaced as well. unless, if it was a mis-labeled gas neck, would that have anything to do with venting properly?

 

[vail426]

I think the cap lets the tank vent when necessary and also will allow pressure on the other hand. I would think a slight pressure would be more inclined to prevent foaming. This is how I understand it or I may be off base.

 

[VW_Lupo_TD]

you dont want hissing

how can it not hiss when opening if there is a vacuum ?
there has to be some sound when you open it.
and yes, there is supposed to be a vacuum.