i go to the gas station today i already had about 1/4 tank of gas, and when i unscrewed the cap, it started hissing like it was releasing pressure or something. I topped off the tank and drove it home. When i came back out about 30 min. later, there was a nice big wet spot (gas) under the back of the car. It seems to be leaking down the shield thing from the tank. It was a rather large spot too. i drove it around some more, and wasted about a 1/4 tank of gas, came back home shut the car off, came back to look at it in another half hour, and theres nothing there. Whats going on here?

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My 1986 Z28 and more!!
85 IROC w/ 1364 miles!

R.I.P Dale Earnhardt

 

fuel tank vent isnt functioning properly, Check your charcoal canister.

its possible that enough vaccum built up to actually damage your tank.




------------------
60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver

Just another Hot Rod kid, or thats what they all tell me.
Livin' the Stereotype

 

The tank is supposed to be pressurized. In fact, on most newer cars, not having pressure in the tank will set a code; when you look it up it will say "gas cap loose" or words to that effect.

If you have gas leaking from under your car, you have a problem. Check all the rubber lines that connect the gas tank to the car, in front of the rear axle just inside of the left rear tire.

------------------
"So many Mustangs, so little time..."
ICON Motorsports

 

The "hissing" is normal! Mine does that all the time! Nothin' wrong with that! The leak is another thing all together!! Check your fuel line for leaks!!

 

i checked all the lines etc.. nothing there. so i followed the leak up and it is coming right out of the seam where the filler neck meets the tank, which would probably explain why it only leaks when the tank is full. so are we all sure that the hissing is OK? one person here said something about a vaccum valve or something being bad and causing enough pressure to actually cause a leak (ie- seperate the seam on the neck)? Can anyone else back this up?
thanks for everyones help so far...

------------------
My 1986 Z28 and more!!
85 IROC w/ 1364 miles!

R.I.P Dale Earnhardt

 

All I'm sayin' is that mine hisses just like you're talkin' about and it has for a LONG time now and I don't have any leaks in my gas tank!! Maybe your tank is just worn out!!

 

the "hissing" is just the excess pressure releasing itself out of the system

 

The hissing is normal. The tanks run on a pressurized system. As for the gas leak the weld on the filler neck where it goes into the tank is probably bad. I'm pretty sure there was a recall on the 91-92 camaro's bacause of a weak weld on the filler neck. Call a GM dealer they would know.

 

The leak you are talking about where the filler meets the tank is pretty common and GM had a recall on certain years and put in a new tank.I do not know whether the recall is still valid but worth checking out. You probably should replace the tank with a new one if you have a leak there...........bob

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91 camaro RS L03 convert,
t-5, ultimate TBI mods,TBI spacer,SLP 1 5/8 headers,
3"cat back, 3.42 torsen posi,alston SFC's,Global West LCA relocation brackets, edelbrock STB

 

It's a snake! Kill it!

I ended up putting that epoxy putty on the filler neck in mine, it seemed to do the trick.

 

There is a recall on 1992 Camaro's for a leaking gas tank. Dealership will require you to bring the car in with less than 1/4 of a tank. They bascially pressurize the tank and make sure it holds air. Not sure what year your car is.

------------------
Rob P
89RSconvtZZ4TPI
Edelbrock Hi-Flow Intake
SLP Dual Cold Air Intake
1 5/8" Headers
Semi-Siamesed Runners
MSD6AL/relocated MAT/ AdjFPR/IROC frnt@rear swaybar/wonderbar/steeringbox/alum drvshaft/ Alston SFC/3:23posi disc rear/MAC LCA/H.Adams Panhard Rod/KYB struts/ shocks/
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!

 

i guess the recall wouldn't apply with me, it an 86' thanks anyway though


------------------
My 1986 Z28 and more!!
85 IROC w/ 1364 miles!

R.I.P Dale Earnhardt

 

Mine doesn't hiss... and I was told THAT is normal. Is it different for FI cars to hiss? I think my rice does...

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

Mine hisses. I hope it's supposed to because it has been doing it since I bought the car from my mechanic friend. So far everything's ok...

------------------
1987 Camaro Sport Coupe LG4
700R4 with 2.73 open rear
Hypertech chip, 180* stat
Dual snorkel air cleaner with K&N air filter
CK rods with B hanger
Hooker catback
Pioneer DEH-P4000, Pioneer speakers front/back

 

Carbed cars don't "hiss" like that!! My friend had TBI and his "hissed" but after he switched to a carb it quit!! I think it has somethin' to do with the EFI fuel pump being IN the gas tank!!

 

My 83 HO car has done that for as long as I have had it, since 85. Carbed 3rd gens do in fact have pressurized gas tanks just like FI ones, as long as the charcoal canister and tank vent system is left as the factory built it.

------------------
"So many Mustangs, so little time..."
ICON Motorsports

 

Ok, you guys are scaring me. My 85 TA is carbed and it's been hissing like that for the whole three years I've had it but I've never had any problems. If someone knows FOR SURE that this is a problem, please let me know.

------------------
85 TA LG4 Automatic. K&N,Dynomax Super Turbo exhaust,ADS Superchip,160 stat,Hypertech Coil, Moroso Blue Max Wires,Rapidfire plugs. Hey, I know it ain't much, but $$ is hard to come by!

 

My 85 Camaro makes a hissing noise and I hear a thump inside the tank when the gas cap is removed.

 

So are we all in agreement that this is a normal function of carbed, and FI cars? GOOD. that means I have one less thing to worry about....

------------------
My 1986 Z28 and more!!
85 IROC w/ 1364 miles!

R.I.P Dale Earnhardt

 

I believe mine used to hiss with the q-jet... but it stopped when I put on the Holly. What did I screw up then? I'll check the charcol canister lines I guess...

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

Gearhead, that thump is your tank "snappinig" back into shape once the pressure is relieved.

That too is normal (shows you have good pressure). When I had a "motion detector" activated on my car alarm, the tanks "popping" as the car sat and cooled down would trigger the alarm.

I discovered this when I was standing by my car one day after turning it off for an hour and trying to figure out why my car alarm constantly would go off; then I heard the gas tank "pop" and the alarm went off. I talked to the car alarm shop and they said this was a very common problem with many of the new cars today. I then had the "motion sensor" reduced to its lowest sensitivity to stop this problem as my neighbours were about to kill me for my car's alarm waking them up all the time.

 

Ok, I was wrong!! Now that you mentioned the charcoal canister I just realized...my friend hasn't have the charcoal canister since he wrecked his car a few years ago!! Guess it's not givin' him any problems though!!

 

i guess i'll play the idiot roll here,...

what is the charcoal canister and where is it?

yeah i know, stupid question....

 

Charcoal canister, driver's side front corner of the engine bay, black barrel with a couple 3/8" hoses on it and sometimes a solenoid. Quenches the tank vent fumes, also an emissions requirement.
This is kinda funny to me, I spent a couple hours trying to get that hissing back!! I don't have the canister control valve that goes up near the intake, just the tank pressure valve. I finally resorted to just hosing the tank valve straight to the "H" vac port under the TPS. Now it hisses somewhat, but my tank cap may also be fuct.

------------------
DR Stevens
1984 Trans Am 305HO L69
sucks gas, hauls ***
1991 Firebird 305 LO3
for days when I need a/c

 

Zup,

...but it would be a lot less intelligent if you didn't ask the question. It's odd how many things I've learned just by asking about that which I did not understand. It would be pretty dumb NOT to ask when you know the answer is right out there at the end of the modem.

Anyway, there is supposed to be some tolerance for latent pressure in the fuel tank, and only a slight tolerance for vacuum. There is a tank pressure control valve incorporated into the EVAP system (top left corner of the diagram).


If the pressure control valve is faulty (or removed) the tank can exceed its design pressure and rupture, blow off the vent hoses, purge out excess fuel from the filler neck and relief valve in the cap, flood the carburetor, slug the evaporative emission canister with raw fuel, or leak.

The EVAP (charcoal) canister should be located in the right front corner of the engine compartment on your car. There is a filter pad on '86 and older cars that needs to be replaced every 30,000 miles, but seldom gets replaced:


------------------
Later,
Vader
------------------
"I'm'a do Things My Way - It's My way or the Highway."
Adobe Acrobat Reader

 

Guys (and girls),

maybe I'm stupid, but I don't understand why so many of you have this happening. If Zupmans car is carbed, there shouldn't be any pressure at all in the tank if using a mechanical pump-they pull the fuel from the tank and there is usually no return-so what would make pressure inside the tank? If it's a vacuum hiss, then either the vent system or the cap isn't letting air in to replace the fuel as it's removed. It sounds like pressure though, since it is leaking around a seam.

If it's a tpi system, there still shouldn't be any tank pressure, I don't care how many of your cars' hiss. It shouldn't happen in a proper system. Any pressure-positive or negative-means the pump is working harder than it needs to, or fuel is being forced past the pump. Zupman, do you have a great big aftermarket fuel pump? Maybe it's returning more unused fuel to the tank than the canister can vent. Or maybe the canister is clogged or the vacuum line is plugged-I think someone already suggested that though.

My truck (even though it's TBI) never ever makes any hissing noise at all when I take off the cap.

------------------
'93 2wd truck
408 sb, vortecs, 454 TBI, Comp Cam, dual exhaust

 

I think the almighty and wise Vader has found my problem. The filter in the bottom of my can is black and nasty, i doubt it's ever been replaced. Im gonna go get a new one this afternoon and see if that helps. I guess this would coincide with the tank not being able to vent itself, and causing it to "hiss" out of the filler neck when the cap is off. I'll let you know what happens after i replace it. Oh yeah, my fuel pump is new, but it's nothing spectacular, it's basically an upgraded stock replacement.
thanks for eveyones help on this...

 

Well I'll be damned if I didn't learn somethin' new today! I have an '86 TA and didn't know that filter needed to be replaced!! I've had the car for probably 80,000 miles and never replaced it!! Guess I'll do that today!

[This message has been edited by 86TpiTransAm (edited April 07, 2001).]

 

Some of the fuel in the tank turns into a vapor and causes pressure. Thats why the vented gas caps are gone and charcoal canisters are in the cars, to minimize the amount of fuel vapors emitted by the car from it just sitting there.

 

Madmax is absolutely right. TPI cars should not have vented gas caps, all fuel vapors are absorbed by the charcoal canister and then the canister is purged into the intake by engine vacuum using an ECM controlled solenoid. The objective of the system is to keep fuel vapors (evaporative emissions) out of the environment. Pressure builds up in the system easily, especially on warm days, and also after you run the car for awhile and then shut down since the fuel system uses the gas tank as a heat sink the whole time you are running with a TPI car, thereby heating the gas and making some of it evaporate until the air in the tank is saturated. When you pull the cap on a TPI car, especially if the weather is warm, it had damn sure better hiss a little or you have a leak somewhere.

------------------
1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, Accel coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Ruger P95DC, hot wife, new oil filter,
thick rubber floormats, towels in the seats.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

Hey PhantomChamp,
Tell me more about putting epoxy stuff around that fillerneck!
Did you had to remove the tank,or did you put it on while the tank was still under the car?
The fillerneck on my '88 Firebird is leaking as well,and I'll try to stop it the same way you did!
There is a recall,but only for 89-92 cars!
My 88 is not coverred!
I contacted the GM service dept. many times on this VERY dangerous situation,but they don't care...
I think they(GM) didn't learn from the Malibu incident!
So many thirdgens with leaking fuel tanks is just a bad design!!
All affected cars should get new tanks i.m.o.!!

 

You will find that unless you get your cap from the dealership for about $8, it will tend to leak. I had that problem.

------------------
'92 Z28,t-top,PPG jet black,5.0L TPI,Auto w/stage 2 sft. kit,
Z rated tires,polished plenum/intake,flow tech 3" exhaust,PIAA driving lights

 

My old 91 Z28 5.7 used to THUMP after you shut it off, parked it and walked away.

------------------
Rob P
89RSconvtZZ4TPI
Edelbrock Hi-Flow Intake
SLP Dual Cold Air Intake
1 5/8" Headers
Semi-Siamesed Runners
MSD6AL/relocated MAT/ AdjFPR/IROC frnt@rear swaybar/wonderbar/steeringbox/alum drvshaft/ Alston SFC/3:23posi disc rear/MAC LCA/H.Adams Panhard Rod/KYB struts/ shocks/
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!

 

was that recall 89 or 90 ??
I thought is started in 90 ???