I have been fighting this issue for two summers. I have a ton of pressure in the gas tank after I fill it. I drive about 8 miles and I smell gas and it is leaking out of the cap and running down the side of the car. If I remove the cap, gas will shoot out onto the drive until the pressure is released. It does this until the tank is down to 3/4 full. It is a brand new tank and a brand new fuel pump. I have taken it to the garage and they cant figure it out. Again, they have been working on this since last summer. They thought it was the vent line but I can blow air in the vent line all the way to the filter in the engine bay. I am at a complete loss. It there a check valve on the sending unit or something? Any help would be great!! Thanks! Tim

What year, what engine?

There is a separate pressure valve on many 3rd gens near the tank (as opposed to a vented gas cap that many other cars use). It's supposed to vent at about 1PSI of tank pressure to prevent damage to the tank and other issues similar to what you are describing.

In short, some pressure when removing the cap is normal, excessive pressure is not.

There's an external relief as Damon said, over rearend just follow the lines from the tank and you'll run into it. There should also be one on the flange where the lines go through into the tank. The other thing would be to check the Charcoal Canister for proper operation.
If you need pictures i can send them to you, otherwise I'd have to search and insert a link to it, in that case you can do the search to find them.

Its a 305 V-8 on a 91 RS. No vented cap. I definately would need the pictures if you have them. I know zero about cars. Is the valve in line on the passenger side by the rear wheel well? I have a ton of pressure as i said, it shoots gas right out of the tank, when its an empty tank then I have that little hiss, but when its full...watch out that tank blow gas all down the side of the car right out of the filler cap.

Also is there a check valve in the cannister?

I finally figured it out. It was the check valve on the Charcoal cannister. It is not working properly. I ahve bypassed the check valve and I am venting directly into the cannister. Thanks guys for all the help. Hopefully I can find a new check valve or cannister assembly....

is there anything that could cause this because i am having same problems

I thought mine was fixed but now I am having the same problem all over again. And by by-passing the charcoal cannister it just made the car hesitate at the end of the tank. I just filled it, drove 4.2 miles and now its blowing gas out of the filler cap all over again. I am at a loss. The only thing next is to replace the whole fuel system. I just cant imagine after two new fuel tanks that its the fuel tank, but who knows???

Does anyone have any idea's??

I just took the charcoal canister out of mine completely and left the vent line open.

There are two ways that pressure is relieved from the tank: a pressure relief valve over the rear axle on the drivers side (Off white in color, and made of plastic) and the purge valve on top of your charcoal cannister. The most likely cause is the relief valve. To test it, remove it from the car and blow as hard as you can into the side that had the hose attached to it. It should "Vent" or "Blow off". If you cant get it to do it and no air comes out, then it is defective and should be replaced. I have a part number somewhere.... I will look it up for you, if you like.

What about the reburn line, What did you do with that...and do you get a gas smell?

I will try that next...If you get a chance could you please look up that number. I dont have any faith GM will have it but it would be helpful.

Your local GM dealer should have it. part number is: 10033125. I just picked one up at Power chevrolet in AZ, in march.
Let me know if you have any problems finding it.
I am sure this will fix your problem.

Ok so i have replaced both the canister and the check valve over the rear whell and I still have a TON of pressure in the tank. I drove it about 4 miles and it just starts leaking out the filler cap again. If i take the cap off, there is enough pressure to shoot about a gallon of gas out onto the driveway. I am frustrated and confussed. I love the car and I dont want to get rid of it. Could it be a sending unit issue? A tank issue? or do I need all new fule lines and vent lines?

Bump

WOW!! Now I would have to say it's a blocked line! The check valve over the axle will vent when pressure builds up. Check that line. You can hook compressed air up to it and blow a little in (Not a lot, no more than 20psi) If you take the gas cap off first, the air should blow right out the filler neck. Maybe the sender has a blocked line....

I just wanted to thank everyone for all the advice, and pm's. This really is a great site for help especially when your local mechanic doesnt even know. Any way, just to keep everyone posted i will be dropping the tank hopefully this week, everyone knows how much fun that is - to see if there is a problem with the sending unit. Thanks again everyone.

Sounds like a photo shoot opportunity for a tech article, and or sticky to me. I sure many would love to see how it's done.

Its fixed, or at least it seems to be fixed!! I finally got around to dropping the tank. I replaced the sending unit and put the factory gas cap back on and now I am leak free. So after all that I replaced, which was almost everything, it was the sending unit the entire time, as well as the OE replacement gas cap. After the sending unit was replaced it still had a minor leak out of the filler cap. I put a new O-ring on the factory gas cap and that stopped the leak from the filler neck. Thank you to everyone who replied for all your help and idea's. Lesson learned if you replace your gas tank, replace the sending unit.

So after everything I did, I am back to square one. I have gas leaking out of the filler neck at the cap again. Has anyone heard of a problem with an after market tank not sealing properly to the original or eo equivalent cap? I am at a loss and that the only thin I can think of. HELP!!!

this is an old thread but has anyone figured this problem out

just had the issue on my car but my system is now setup for a carb. new aluminum line from tank to mechanical fuel pump. Filled it up yesterday lost around 7 gallons of gas on the ground. Found out my fuel filter was clogged and after changing that no issues. Also if you use the fuel pumps that have to seal to the filler neck and you have to push them to start pumping, they can create a syphining effect.

Hey guys I was also having this problem when I first bought my 89 RS 1 year ago. What you may need to do is make sure that the cap is going on correctly. It should not be tight after just 1/2 a turn. The threads need to line up and should make atleast 1 full revolution before any clicking occurs. If I did not do this properly I notice that the pressure builds up and also that it will spill out the tank. The threads aren't lining up causing pressure issues and leakage.

Alex

vapor lock....the 10% ethanol in the gas has a quicker boiling point then regular gas.july and aug are the worst.your gas is vaporizing in the tank and the car shuts down or spits out gas.when the car cools the problem goes away.This happens mostly in the summer you say?that is my problem,everybodys problem!