So on my 89 GTA, if I fill the fuel tank past 3/4 of the way full, after going for a drive I will notice that fuel has literally spilled out of the filler neck. As if it's bubbled out or something. I've tried three different gas caps and none fix the problem. Any ideas what in the world would cause this. Obviously it's not too big of a problem because I can just fill the car a little less than 3/4 everytime I fill up. But this is still frustrating nonetheless.

 

Sounds like your Charcoal Canister is not venting. I had this problem when my line got pinched.

 

Thanks for the input! Any idea how I'd go about troubleshooting it? Someone said it could be caused by too much pressure from an aftermarket fuel pump but that just didn't sound right.

 

Well an aftermarket pump at a higher rating is creating more pressure but your vent line and charcoal canister must be both clogged or pinched.When you open your gas cap..does it sound like its sucking in air or hissing pressure out?

And usually you would smell fuel vapors all the time inside the cabin aswell.The car drives fine right?No bucking at higher rpm or anything unusual?

 

Car drives perfectly fine. And opening the gas cap doens't release a hissing yet there is always an abundance of fumes around the gas cap. Even with it shut all the way, regardless how much gas is in the tank. Yet, the smell never gets inside the car. So where is the filter on the canister? Do I take the canister out of the car to access it?

 

Still really confused:

Bought the filter for the canister but it turns out mine is the non-servicable type and I didn't feel like dropping 150 dollars on a new one. So I unplug mine and cap all the lines, And I'm STILL spewing fuel :X

 

There should b a check valve down in the neck or a slosh door or something like that covering the nozzle hole..

 

Are you talking about the silver flap over the filler door hole? Cuz that's there. Or did you mean somehong else?

 

Sounds like the gas cap isn't sealing. In the summer after driving the car there should nearly always be pressure in the tank. When I fuel up there is a lot of hissing when removing the cap.

Check the inside of the gas cap for the rubber seal, make sure that it is there and in good condition.

Check the mating surface on the filler neck. Shouldn't have a nicks or gouges in it.

Lastly, it may be the cap itself. When purchased the '92 wouldn't always seal. I found that the cap wouldn't always thread all of the way onto/into the filler neck before clicking. It was worse in hot weather and the fumes were bad. The cap clicked so early that it actually held the door open some.

To fix it I used a rat-tail file and filed the grooved portion of the cap threads smaller. Then used a flat file and filed the tops off the cap threads. Not a lot, but enough that the cap was easy to turn into place.

Problem solved.

You should re-connect the CCP lines as that prevents an excess of pressure to build up in the tank. The vapor control valve has a low cracking pressure, while the tank vent valve has a much higher cracking pressure.

RBob.

 

Thanks for all that information! I just checked everything you said and switched to a new gascap today. And It's still spewing fuel! This is so aggravating because I filled up with a full tank so after everytime I go for a drive I'm going to have to wipe fuel off my car for the next 50 miles.

 

Wow very odd! I'm thinking there is a one way type valve in the neck down in the tank, maybe_maybe not.. Ill check some tanks here and see...

 

If you have some modeling clay or play-doh, place a small piece on the filler neck where the cap seals. Then thread the cap down. Remove cap and check the clay/dough, it should be well flattened. If not the cap isn't going all of the way on.

More or less it has to be the cap to tank seal.

Just a side note, our cars take a non-vented cap. Something else to check.

RBob.

 

Well the cap I'm using now is a brand new, non-vented, non-locking cap. It feels like it goes all the way down and has a secure seal. But I haven't done the clay test yet.

 

The line from the tank to the charcoal canister should not be plugged. When plugged, it will allow a large amount of pressure to build inside the tank. I found this out because my line was pinched closed by accident.

 

So even if I remove the canister, the line should still be venting? Maybe I'll hook the canister back up and try with this new cap. I'm becoming so irritated with this thing. Lol.

 

The cracking pressure of the CCP vapor valve is much lower then the other tank vent.

As for the leaking, while filling up today I started to think about how the upper part of the filler neck is assembled. So when I got home took a better look.

It appears that there is another path that the fuel can leak from. The portion of the filler neck where the cap seals appears to be pressed, or soldered, or even maybe glued into the neck.

So the gas may be leaking out from between this piece and the filler neck.

The piece I am referring to also has the little filler neck door/restriction.

RBob.

 

If you have the proper cap for the application (I sure hope so with all the caps you've installed!!) then it might be a crack in the filler neck. They sometimes get broken when dropping the tank to install a new pump. GM even had a flexible filler neck section installed under a TSB years ago to repair this kind of problem, if memory serves.

 

Well, after reading all the GREAT information (thanks!) that everyone gave here, I troubleshooted and checked everything I was told to do on this site. Still found NO problems! So I said, what the heck, might as well give one more gas cap a shot. I went to Autozone and they gave me a brand of cap I've never had before, CST, and it feels like it seals tighter and better than any of the other caps have. Been driving about 20-30 miles and no fuel spew! So glad, afterall this trouble, it might've just been a bad string of caps. I'll post another update in a few days and let everyone know if this worked or not.

 

Oh wow.. glad you may have something figured out!
I believe stant made them for GM but I could be waaay wrong.

 

All the other ones I had were Stant...strangley enough. I thought they're like one of the best cap brands around lol. But it turns out, the first two were vented, and these arent suppossed to be vented, and the third one was a locking one that just didn't quite go far down enough to the rubber seal on the cap. On this new one, it is a tight seal and non-venteed. So far so good. Keeping my fingers crossed. Once again, I GREATLY appreciate all the help and input from everyone.