I have completed installation of a new 383 engine in my ' 72 base motor vert, and in the processess have re-attached all vaccume lines associated with the carbon (charcoal?) canister. If I understand the way Chevrolet designed this emission system to work, there were three hoses going to a switch on the canister. One went back to the tank, a second was attached to manifold vaccume to activate the switch allowing a third line plumed into the PCV circuit to suck the fumes out of the canister into the manifold and thus be burned. My problem is that now, after hooking up all the lines as they should be, the tank seems to have a lot of pressure on it when I take the gas cap off. It's a sealed system, but the pressure seems to be excessive. I remember pumping gas in the 1970's at an Arco station I worked at and some cars did the same thing, but not to the same extent. Has anyone else experienced the same symptoms?
Thanks

Todd

 

Maybe just switch over to an unsealed ("vented") gas tank. The vacuum in the gas tank, with your present system, acts as a hindrance to fuel delivery. I think that the addition to air pollution caused by switching to vented will be insignificant since most likely you wont' be driving your car very much anyhow.

Corvettes prior to 1970 had vented gas tanks. You can buy a vented gas cap for $24.00.

 

Here's my theory - "This may be normal".

In the warm summer months, the gas in the tank heats up and what you are seeing is the normal vapor pressure of gasoline. If the system is truly air tight, you will get quite a whoosh when you open it up.

Don't know if there is anything you can do about it. Was it this way in the winter??

 

Z,

No, as much as I drove it in the winter, I never saw this before but at that point I hadn't put in the new engine or re-attached the vaccume hoses for the canister either. That's why I am wondering if something is wrong with the canister. After releasing the pressure in the tank by taking the gas cap off and then running the engine for 1-2 mins it builds right back up again. Weird. I don't know if this is normal.
Todd

 

I see - that sounds excessive. I'm not familiar with the cannister systems although I've heard that they can plug up over time...

 

Sounds perfectly normal to me. My '73 has been like that since 1990 when I bought it, both pre and post frame off restoration.

 

No, its not normal to have pressure in the tank as it has a filtered vent through that charcoal canister. No switches either - a passive system.
Ok the long story. The tank vents through a liquid separator on the drivers top side of tank - this keeps liquid out of the vent line to canister. Pressure and vapor vent down that drivers side line to connect to one of the 2 connections on top the EVC (stock EVC not NAPA replacement with 3). Also that stock EVC has a bottom vent to atmosphere through the charcoal and a disposible/replacible cotton filter (NAPA EVC has atms vent on top - no cotton filtter). This EVC will vent any pressure to atms but the charcoal is suppose to trap the fuel vapors (after 30 yrs, yea right ).
Still awake? Ok then that other fitting on top the EVC goes to a ported vac on the carb which exits just above the pri-throttle blades - this is so vapor only enters at a part throttle opening where the rpm can handle the richning vapor - idle speeds can't handle this.
Wheew! Thats it.
So it sounds like ur line or the cannister is somehow plugged or maybe connected to the wrong ported vacuum on the carb. Or even that line is under vacuum (from manifold) and the sound u hear is a neg press/vacuum of air rushing in not out. Possible that liquid separator way back on the gas tank is plugged or seated from vacuum. Also u could have the wrong cap - the non-vented caps had a press relief of 'bout 5psi.
Anyways try and draw the system out to make sense.
BTW NAPA still sells a replacement but i buried that ECV part # somewhere but PM me if u need it - thier web site search didn't show it.
Good luck. cardo0

 

Yawn...

All this complexity just to keep a few molecules of gasoline from excaping into the air.

Switch over to a vented gas tank and forget all of this. I live in LA and I'm glad that the air pollution here has diminished, but I don't think that car gas tank fumes are all that significant.

 

Well...I'm certainly no tree-hugger. Just like the next guy I like running my big-block boat at full throttle through a herd of Manatees and then grilling a nice fillet of Spotted Owl over my charcoal-fired grill, but...

Emissions (including those "few molecules of gasoline") are measured in the BILLIONS of POUNDS annually. Yes, one car's emissions are insignificant, but...there's more than one car in L.A.

 

Whoa, Nellie. Billiions per year. How many billions? Two billion? 100 billion? A million billion. I think that people who have a hobby of restoring their C3 Corvettes can drive around with unvented fuel tanks without concern that they are going to create a horrible environmental problem.

 

We're not creating the problem, just adding (a little) to it...

http://www.scorecard.org/ranking/ran..._state_code=06

 

I totally agree with the above, but to answer the guy's question....

I have had my '72 here for nearly ten years now, and have noticed when the tank if pressured, that the float ball of the vapor recovery system has been floated to the top, and stuck there, not allowing gases to escape, so hot gasoline from the return line goes into the tank as well as it getting heated by the exhaust system....so the vapors are blocked from the recovery charcoal tank....along with raw fuel in the first place, but the tank is then sealed up tight, in other words, the design is messed up from the git go....
what I do is just ignored the main vacuum line input to the top of my recovery canister, upon noticing it was not working, the valve is in the open position venting to the ported vacuum side of the t-body (I have DPFI, not that it matters for this discussion)....this allows the minor vacuum to suck the fumes, I find this works much better and rarely find any pressure in the tank these days....

GENE

 

Lately I've been working on my 1968. When I get that finished I'll start on my 1970 BB Coupe. It has that vapor return charcoal cannister and all the other stuff (A/C,PS, PB, PW,T/T, factory alarm, leather seats, shoulder belts. Delco AM/FM stereo). All of this stuff will be restored to operation, however the vapor return system will not be working. I will have a vented fuel tank!!!!!

........PS i bought the 1970 Corvette 454BB in 1992 on something of take it or leave it attitude. It was a nice car, but I was then a little bit indifferent as to whether I really wanted it. I was then thinking that I should buy a C4. Today I'm so glad I bought the 1970 BB Coupe. It's a great Corvette. I think it's a little bit of a rare car. I don't think there's that many similar cars in existence. Low rpm torque is just amazing for a street car. I paid $15,000 for it in 1992 (the seller disclosed that the engine number did not match). I later thought that was a little bit too much. By todays prices, it may not be all that bad.

 

I've had a similar problem, I eliminated all the emissions comntrol from my '73 when I completely re-did the whole car, including the engine. But in the last year when I fill the gas tank I get a drip on the ground caused by a broken fuel seperator on the gas tank. I took some advice and eliminated it, however when the ambient temp reached about 85 or 90 degress in the garage, pressure increased in the tank enough to force gasoline past the check valves in the carb. Without giving you all the gory details of what I've been through since that happened over the course of 5 days, let me just say I've drained 10 quarts of gas/oil mixture out of the crankcase. My advice-replace the seperater or buy a vented gas cap so pressure can'r build up and cause this problem!
Syl/Gateway City

 

Would just switching from a sealed cap to a vented cap solve this issue of pressure build up and bypass the can in the eng bay?
Gary