Gas cap

[79TUFF]

So i replaced an after market gas cap with a new GM stock one because it wold not vent vapors through the cap but the new one does the same thing. Am i wrong to assume they are not supposed to hisssss when opened?

 

[TotalyHucked]

What year truck? The tank should be vented, not the cap...

 

[PrairieDrifter]

^^^^ the tanks should be vented. Not the cap.

 

[Don-T]

Mine did the same when I bought it. I bought a vapor canister and installed it per information on this site. No more trouble with gas caps.

 

[79TUFF]

Suburban is a 1990 model, i tried unsuccessfully with the search on here, i will try the vapor canister route, thanks Don.

 

[Don-T]

Try the link below to get you to some good information. First time pasting a link within the forum so I don’t know if it will work.

Charcoal / vapor / evap canister, how it should be set up and other information

Posts 1, 2, 3, 4 & 6 are my general information posts, they are all geared towards carbureted rigs, the principles are the same for fuel injected rigs, but the controls will be different. So yesterday I installed a new Charcoal Canister and did a post in another thread and went on a bit...

www.gmsquarebody.com

 

[TotalyHucked]

Suburban is a 1990 model, i tried unsuccessfully with the search on here, i will try the vapor canister route, thanks Don.

You should have a factory vapor canister under the hood. Get under it and trace the lines from the tank to the vapor canister and make sure there's not pinches/restrictions. You can try removing the line on the vapor canister for a couple days and see if the issue goes away, you'll just smell fuel. If that works, your vapor canister is probably no good/full of fuel.

If that's not the case, you may have to drop the tank and investigate further. You should have a supply line, return line and a vent line in the sending unit. If you don't care about having your factory vapor canister stuff hooked up, you can always just run a new piece of hose out to the back bumper with a rollover valve on the end. That's how I've got my truck (just straight hose, no rollover valve but that does help with fuel smell when stored in a small space).

 

[Ricko1966]

The problem with vented caps and tanks not vented through the charcoal cannister is water absorption by the fuel,rusted gas 5anks from the water,frozen fuel lines from the water as well as the gas fumes in your garage and your gas evaporating while parked,the more aromatic parts evaporate furst leaving you with turpentine for fuel( I prefer to burn my fuel). Take your lawnmower can fill it half full leave the vent open and leave it on your porch for a month,see what happens. Do the same thing but close the vent,see what happens. There's all kinds of reasons manufacturers quit venting fuel tanks into the atmosphere.

 

[79TUFF]

Thanks for the info, i will start at the vacuum canister. I do know that the previous owner had the fuel tank changed, hopefully i dont have to drop the tank.

 

[Ricko1966]

Thanks for the info, i will start at the vacuum canister. I do know that the previous owner had the fuel tank changed, hopefully i dont have to drop the tank.

Do you have no drivabilty problems,just a sound when you open the tank? That may be perfectly normal,and a good sign the Evap system has no leaks. That sound is some of the gas that was trying to escape but couldn't until you opened the cap. But think how much hasn't escaped over time. If it's building presdure,but not pulling a vacuum,I wouldn't be worried about it. If it is pulling a vacuum,you'll know it,it will starve for fuel because the fuel pump can't overcome the vacuum to draw fuel.

 

[79TUFF]

That makes perfect sense, truck runs good otherwise. I had an 87 w/ tbi and the cap never held pressure like this one.

 

[gmbellew]

My 90 burb TBI has no pressure releasing when I open the gas cap. I'd check out cannister under the hood to make sure it is all connected right and that the vent line from the tank isn't plugged. I think there was a thread on here somewhere about the cannister set up. It's also covered in the drivability and emissions manual.

 

[JBswth]

So i replaced an after market gas cap with a new GM stock one because it wold not vent vapors through the cap but the new one does the same thing. Am i wrong to assume they are not supposed to hisssss when opened?

All 1970 and newer have non-vented caps, or are supposed to. The tanks are supposed to be vented via the Evap system.

J. B.