Please explain how the fuel tank vent fitting works? 84 Colony Park

[drhill]

I'm trying to get to the bottom of my gas tank pressurizing. For the last few years I just left the cap slightly loose and no issues but occasionally when the tank is mostly full I can smell raw fuel driving down the road with the windows up. I have the oem tank vent hose connected to a small round air filter(like you would use for a valve cover breather) and positioned just above the top of the gas tank - no oem evap charcoal cannister or purge valve anymore. It's got a turbo LS motor so the factory stuff is long gone.

There is no evidence of fuel leaking around any of the seals on the tank, so I tried to blow through the tank vent hose with the gas cap tightened all the way. I was able to build pressure in the tank, that released only when the gas cap was opened. I guess I either have a clogged tank vent fitting or I don't understand how it works? I would think that I wouldn't be able to build pressure in the tank because that hose should be open to flow both ways right?

thanks for help!

[sly]

The tank vent has rollover leak prevention in it (a ball that fills the hole to slow/stop fuel leaking out when upside down). Sometimes the ball gets gunked up and the vent gets clogged.

Quick question though... are you sure it's pressure and not vacuum?

The cap should allow air in but not out. The vent should allow pressure out but not in IIRC. That keeps the system one way only. But yeah, if the vent is plugged up... It will pressurize due to fuel evaporation and will make filling a PITA.

Not sure when the cap changed, but maybe the older ones seal both ways and only the vent lets pressure out. But yeah, if you're running a turbo LS, you should probably make sure the cap lets air in.

[gadget73]

What he said. usually its something in that valve at the top not working. I've heard of people taking them apart and removing the check ball and slightly enlarging the hole to allow it to vent better.

but yeah vapor out through the thing in the top of the tank which runs up to the carbon canister, air in through the cap.

[drhill]

got it, thx guys. I'll drop the tank and check that valve. I have the original gas cap and it appears to seal completely but I'll look at it closely. I've been crutching the tank vent by just cracking the cap open so no pressure builds....it also works to allow more air into the tank when I'm racing because of the sheer volume of fuel I'm using (e85 at approx 1000 rwhp) is more than the stock setup was designed to accommodate in terms of vacuum in the tank from the huge fuel pumps I'm using. It would be nice to close that cap tight when I'm just cruising on the street knowing the vent valve will just do it's thing. And, if I still get smell I'll run that line all the way up to the motor through a homemade charcoal cannister