this situation is really pissing me off...every so often after i drive the car i hear a loud "BLUMP" comming from the gas tank (the sound of a big metal container expanding/contracting). Ive replaced the vent under the car, and i could blow through that hose meaning the hose connecting to the vent isnt clogged. I also purchased a vented cap a while back. why the heck wont my gas tank vent??? when i unscrew the cap it will make that blumping noise, sometimes pressure comes out (hissing noise), sometimes it doesnt. I just got the vent and ive driven the car 15 miles with it and i just heard the tank make that noise, this time the cap made a hissing noise when i unscrewed it like it should. this tells me the tank is venting....but why is it making this noise!?

btw it wont make that noise if i drive with the cap off

Is there an EVAP canister? Is the tank pressure valve intact (the one for the canister)?

yeah the Evap canister is there, where is the tank pressure valve located? thanks for posting

would too much pressure/too much vacuum in the tank cause any drivability issues? My car has a ton of hissing when I take my cap off (cant tell if its sucking air in or letting it out, I think its letting it in) Sorry this doesnt help the topic creator, but it's nearly the same issue, just without the "babump" noise.

my car does the same thing and I dont even run an evap canister. My vent line ends right near the rear axle.

how would a problem with the EVAP cause this problem? hopefully its something wrong with the EVAP because ive fixed everything vent wise...

Ive been looking for the answer to this myself. My car has an abnormally huge amount of air rushing out when i unscrew my gas cap. The fuel pump is really loud after about 15 minutes of driving when the tank builds pressure. The car is very sluggish and down on power when it happens. I can drive all day long with the cap not screwed all the way on and its fine. I just dont want to for fear of losing gas to evaporation or theives. Is there a way to test the charcoal canister? Everythings hooked up the way it came from the factory.

TTT

does anyone know of some other F-body forums that might be of help?

does the vent line going from the evap can have any restriction.. (pull it off and check) . Make sure it's not plugged up.

Actually this is a chronic design problem with the 3rd gen due to the muffler being located directly below the fuel tank. Free flowing mufflers such as Flowmaster etc make this problem much worse because they give off more heat. Check "Firebird History" and you will find the 305 HO L69 was discontinued in 1986 due to "fuel boiling".
Check your exhaust system and that the heat shield under the fuel tank is still there.

sorry, just jumping in. I have had the same problem so I installed a vented cap a year ago and forgot about it. As for what "Supervisor42" said I have flowmaster exhaust , but I do have the heat shield. If I put the original gas cap back on(non vented) it goes back to its old routine. I'm very interested myself on how to fix this as for the canister, does it fill up? damn! now that I think about it I'm not sure if I know what exactly the canister is or where its at.

The heat shield is INADEQUATE, especially with a Flowmaster muffler. The problem is the overheated gasoline in the tank after a hard run. When the car is parked the heat soaks upward. If the gasoline boils it will have to go somewhere. It is more than the charcoal canister can handle. The factory muffler had some insulation in it but how many 20+ year old 3rd gens still have the original muffler? The only long term solution is to move the muffler to another location that is not under the tank or put heat cloth on the heat shield to stop the muffler from heating the tank.

hmmm.... we have so many options on where to put it..... damn if thats really the case f'k it. Vented cap never seemed so good.

since its not a venting problem, to fix this i tighten the cap just enough so i wont get a lot of fuel evaporation out of the tank, but loose enough to let pressure out....its a compromise until i figure out a better heat shield

ANSWER: Your vent system is working as intended. It is SUPPOSED to hold a certain amount of pressure without allowing it to escape to the atmosphere and becoming pollution. If I recall correctly, systems from the ~1980-1990 were supposed to hold back about 1PSI of tank pressure before venting to atmosphere. (Later systems it's closer to 2 PSI)

1 PSI doesn't sound like much but consider the HUGE surface area of the gas tank. It puffs it up quite a bit and I can certainly believe that in certain situations it can make some odd "burping" noises. Most definitely you can get a heluva rush of air escaping the gas cap on a hot day after driving for a while. Fuel tank fill leve can affect this and make it happen to a greater or lesser degree.

You want NO pressure in the tank? Remove the vent you just installed and leave the end of the hose open. Guaranteed, it'll never build pressure in the tank from that day forward. I would definitely recommend against doing this for multiple reasons, but it's possible.

I've been having the same problem it sounds like I have little people stomping my gas tank. I guess I will try and get the vented cap to see if that works for me.

Jay

i know the fuel system isnt supposed to vent out to the air. i know its pollution. i'd rather leave the cap slightly loose than bust a seal in my tank. if i took off the vent i would start to loose fuel through evaporation....with my way i can tighten the cap when i park the car. Yes i know its supposed to vent out on a hot day like most cars, its just excessive on mine.

Based on my local Chevy dealer (Parts guy) the gas tank vent (check valve) is discontinued and no longer available. Part# 10029752. This "vent" is basically a check valve that only lets air into the gas tank. Therefore, I have done some investigation and found a replacement.

Liquid/Gas Check Valves - US Plastic Corporation

Just be sure that it is put on in the correct direction. Air flow into the gas tank.

i got a new one at my dealer... yes i did get ripped off but it was exactly like the original one

Hey how do these thing (vents) really work. I took my original GM one off and I could blow air through both ways. blowing air through in the direction going into the tank was easy. It was harder to blow air in the direction out of the tank. I also took the original one apart and it looks like there is a spring loaded valve in there that would relieve pressure in the tank if it got to high. I don't think these are just one way vents as earlier stated. Can someone verify this with a new one.

im pretty sure yours is defective...btw i took mine apart too and its only supposed to vent in one direction

if u want i can find the reciept for the vent and tell u how much mine was, i cant remeber exactly

Here is what mine looks like.

http://www.thirdgen.org/techboard/at...082705-003.jpg

More information. After checking into this more I found out that the gas tank vent does 2 things.

1. It will all air to easily pass into the tank as fuel is sucked out in order to keep the tank from collapsing.

2. It also ensures that the tank does not get too much pressure inside. I did not have a way to accurately test this, but if you blow hard enough on the tank side of the vent it will resist for at first and when the pressure build it will give way and let it release. I assume this is a safety feature.

Also i found out that early Part# 10029752 was replaced by Part# 10033125
They are the exact same part just different colors.

i couldnt blow through the new one i bought and my tank still makes the noise and fuel hisses out when i unscrew the cap

I was able to blow through my new one. You have to blow really hard and it will let loose all at once. It is a spring loaded valve. Point is that it will keep pressure in the tank in order to make the entire fuel vapor recycling (canister) system work. So you will get air hissing when you take the gas cap off if everything is working properly. I'm just pointing out that the vent also acts as a relief valve if the pressure gets too high in the tank. I don't know what that max pressure is because I don't have a way to test it.

Well in my case anyway. I read on here somewhere about the muffler getting hot and heating up the gas. It makes sense as to why it builds so much pressure in there. So, i finally got around to putting my heat shield back on between the gas tank and muffler. Its been off for years since i changed the fuel pump. Anyway, now when i drive i get ZERO fuel pump whine and NO pressure buildup in the tank. No air hissing out or anything. I wish i wouldve put it back on when i was done changing fuel pumps. This probably wont work for everybody but im happy to have a normal car again. I hope this helps somebody.

Hell, if you think it's the heat from the muffler, cut a piece of fiberglass insulation and stick it between the muffler and heatshield. If it takes longer to get your 'bump' you know your problem. Then you get to buy some sheet al and cut and install a nice little air deflector to cool the area better. If it doesn't fix it, then look to the system itself.

I wonder, instead of buying a new charcoal canister, is there a way for me to test the solonoid to see if it still works? I'm new and not very good w/ electronics and I now have a digital volt-ohm meter and would like to put it to use.

I had an Edelbrock 67gph pump installed, and when I got it back I had an intermittentant hum in the tank. I stopped it by putting on a vented cap, but I'd like to actually fix the issue. I'm gonna check the rubber lines and the gas tank vent (UFO) and maybe the cannister too.

He wasn't cheap and he bent my rocker panels too.....scrub....oh well...
Eric