When I release my gas cap I get a whoosh like somebody shook a can a of soda (every time) my understanding is the gas tanks are vented (79rx-7) if so why the vacuum? Is it normal? Is there a potential for problems? and should I even care if not?

 

The exact same thing happens on my '82. I have no idea what the answer is. lol...

 

I believe that this has something to do with the charcoal cannister (evaporative emissions) system. Sorry I can't give more detail, but a search will probably bring up what you need.

 

same thing happens on my 93 intrepid..

 

my isuzu truck did this, and so does my vert. my 00' corolla doesn't do it though, maybe because it's nice and new?

 

It should not be hissing like that. If there is vaccuum in the tank then the fuel pump has to work harder and wears out faster. I have no idea how you can inexpensively fix it though.

 

Gas tanks are vented to a charcoal canister to filter out the VOC vapors from the fuel; however, the tanks do hold pressure to a point to prevet all of your gas from evaporating on a hot day. The next time it happens, you will notice thats its not a vacuum, rather positive pressure blowing out. More promenant on hot days.

Its normal and will prevent your pricy gas from blowing in the wind!

 

This can also indicate an inoperative "check and cut valve". This is located back by the fuel tank, mounted to the underside of the car above the rear axle. It assists in venting the system, but also has the duty of cutting off the fuel flow from the vent tube in the event the car gets upside down. Some models also have a vent check valve close to the carb, which can also be faulty.

 

I dont have gas cap The only prob I have is that it smells like gas when i take sharp turns :P

 

I remember reading somewhere that the tanks were always kept in a slight vaccuum to reduce the amount of oxygen so there would be less of a danger if the vaper inside somehow ignited...

 

does the car still have the rats nest? When i removed mine i pulled the canister and plugged the line. Instantly noticed what you are describing. Something tells me its not a good thing for the tank to be pressurised so i just pulled the plug back off. If you still have the canister than try to follow the hardline to the tank and see if there are any flattened spots on it. Its unlikely but possible a random bit of gravel or a rock or something could have hit and crimped the line. Hope this helps.

 

Thanks for alll the input, when I installed the turbo years. ago the instructions were to remove the fuel check valve and I did. Although I dont see how this valve is used for anything other then cutting off fuel in a roll over, if you have ever seen it is a pass through tube with a steel ball when turned over the ball drops down the tube shutting off fuel. But for venting? I am not so sure. I have no charcol canister I do not use the stock air cleaner. I do have a reworked rats nest though following the instructions from BAE.

 

its susposed to do that. thats why some stated check your gas cap and see if it will hold pressure when you go through emmissions. lol you people sometimes just crack me up!!!

 

Hey hyper4mance2k I didn’t see anyone stating to check my gas cap I never had an emissions test and wouldn’t pass if I tried I don’t have an air pump or a thermal reactor but I have a rats nest. You crack me up didn’t your mommy tell you if you don’t have anything nice to say don’t say anything. Now go powder your *** and take a time out. I don’t need your critiquing.

 

Um, it was a typo. Notice on the keyboard how the "D" and "S" are close together? He meant to say that states, as in plural, check gas caps to see if they hold pressure, as in to prevent releasing extra fumes from the tank - just as posted above about blowing it into the wind.

So, once again, as stated above the hissing is PRESSURE, not VACUUM.

This is corrent.

Ever accidently leave a gas can out in the sun? Did you notice how its bulging out the sides when you did?

~T.J.

 

lol

 

One important thing to note here, there should NOT be over 3-4 PSI in the tank, as that could cause the carb to flood. And, just like that gas can left in the sun, a gas tank could start to bulge and distort if the system does not properly vent. Dunno bout anyone else, but I sure don't want to have a tank building up 15-20 PSI on a hot day, causing the car to be next to impossible to start, due to flooding.
I have this same issue, and while I have not yet found which item is causing it, I intent to tackle it this weekend. In the meantime, I loosen the cap everytime I park the car with more then 1/2 tank, and tighten it before driving. under 1/2 tank, I just leave it loose.
As I stated before, there is a check and cut valve back by the tank, and a ventilation and check valve by the carb, both of which are between the tank and charcoal canister. (pages 98-99 in my haynes manual) Check those items, and I bet you find one or more are blocked.

 

I know this is messed up, but I thought it was funny when I saw the topic,, because my gf totally freaked out when she went out to the garage and saw a snake coiled up on the hood of my 85 :P It was a cute little thing, hehe

 

Hey, I just solved my problem. Turns out there is a restrictor in the small hose nipple on the charcoal canister. This has a tiny hole thru it, and was plugged with crud. Since I had nothing anywhere near small enough to go thru the hole inside this restrictor, I used an 1/8 drill bit, and drilled it out. Works great now. Anyone wanting try this mod, take my advice, and use vise grips or something to slightly disform the metal fitting, about 1/4 to 1/2 inch from the end. Part way thru the drilling process, the restrictor started slipping down the tube. As the tube is smaller diameter just before the bend, it did get hung there, allowing me to finish drilling it out.
Before I drilled it out, I could almost pop my eyes out trying to blow thru the hose, afterwards, it flows air thru with slight resistance.
The hole in the restrictor is very tiny, like .030 or so. This allows dust, dirt, or even just corrosion to plug it up.

 

I have heard of gas tanks actually crushing under vacuum pressure. My tank is pretty much nuetral in pressure, and I believe that is how it should be. There is no vacuum line running to the evaporative emissions cannister, it only takes the excess vapor pressure and allows it to go through the charcoal cannister before venting it to the atmosphere.

I have also heard that the gas tank needs to be in a vacuum state to avoid ignition of vapors caused by the sparking fuel level sensor in the tank. This is pure, undiluted BS.

 

On Sterling's carb website he states:
"Another tank related situation that can cause inconsistent flow problems is an inability for the tank to breathe. The tank breathes thru the charcoal canister in engine bay, and if that has been removed, or the PCV system has been removed, condensate can sit in the tank braether line and plug it. A tank breathing problem is evident by a "whooshing" sound when you open the fuel cap after a hard run."

I hear the whoosh noise when I open my gas tank.

Is it supposed to be a negative or positive pressure; or no pressure at all? I'm very curious as I sometimes get some acceleration stumbling.

 

From my understanding, pressure should be nuetral.

 

[QUOTE=Kentetsu]I have heard of gas tanks actually crushing under vacuum pressure.
[QUOTE]

Don't you hate when that happens ?

Seriously, I suspect that pressurizing tanks is more common. I can't imagine the physical requirements for a vacuum to occur.

Should we come up with a totally new mod ? A pressure / vacuum gauge plumbed to our gas tanks ?