The 305 in my 82 t/a wa swapped for a 350 by the previous owner but they didnt hook up any emissions stuff. The gas tank builds to much pressure and i can hear it escaping from the releif valve by the rear axel. there is no charcoal canister so i picked one up at a swap meet and hooked it up to manifold vacuum but that doesnt do it there is a purge valve on the canister.Looks like there some kind of valve i see on the diagram under the hood so i guess thats what i need i have no idea how to hook this up to an engine that is stripped down, just a carburated engine w/Eledbrock manifold does anyone out there know how i should hook this up, right now i need to leave the gas cap loose. Any help would be appreciated. Roger

 

It's supposed to have pressure in it. That's its job. It's supposed to keep every possible gasoline molecule in, and let none out.

If it's making that funky noise like something blowing its nose back there by the axle, it's that valve, and it's defective. Replace it.

 

I guess i thought since there is a vent line that comes up to the engine compartment it should be hooked up to a canister?

 

It is supposed to be hooked up to the canister, but there's some kind of check valve in the line. I don't know the details.

Alot of new cars will set a "Check Engine" code for a loose gas cap, because they're supposed to hold pressure. Here in OH, the only parts they actually physically check in the emissions thing are the existence of a cat (not even whether it really works or not, or has anything in it, just that there's one there) and to see that the gas cap holds pressure.

Don't worry about it, it sounds like everything is working like it's suposed to, except maybe you need the little noisemaker valve.

 

The canister is there for vapors from the fuel tank.(one of the reasons anyway). If you take the canister out, you have to take the check valve off the line also and then just vent it out of the fender by the front tire. If that check valve is still on but can't open all the vapors stay in the tank, hence the sound when you take the cap off. We had this problem on our 85 and another thing that we ran into was that the fuel pressure regulator leaked. We took the check valve off, no more problem.

 

The gas tank will build pressure from the gas vapors. As the tank heats, the pressure will grow greater. These vapors must have a place to escape or you'll end up with a ballooned gas tank and possibly even an explosion. Liquid gas isn't your fear... it's the vapors.

Back in "the days" they used to vent the gas tank and let it purge out to the atmosphere. This causes pollution and has potential to be unsafe in standing or slow moving traffic because the fumes can accumulate and ignite.

So now we have purge canisters. The canister has carbon filters to help clean the vapor content and then circulates the vapors through the engine manifold to be burned in the engine, rather than being dumped into the atmosphere. It's a smart idea and won't effect your performance. But it doesn't do it all the time. There is a bypass valve that keeps it from operating at low engine speeds (near idle). I'm not sure how it operates because I've never looked. But I imagine it's simply based on engine manifold pressure. As the pressure drops from the throttle being cracked open it will activate the valve and purge while you're on the move. More modern systems are likely much more sophisticated.