Trying to learn about EEC Gas Tank System

[harrydunn]

Good afternoon--

I'm trying to figure out if I have a vented gas cap and just figured I'd go the easy route and post photos. Also, what does the EEC system look like on the tank if so-equipped? Is there a metal nipple on the top of the tank that runs to a metal nipple on the fuel filler neck? I don't have one. This is for my '72 which I'm still learning about (obviously).

Thanks.

[wilbilt]

I would be interested in some info as well. I have a '71 GMC 2500 and it does not appear to have any sort of evaporative control. I have only had it for about a week, but bought it from the widow of the original owner. She told me they bought it new off the lot here in CA.

I am curious because the cap that is on it appears to be non-vented, although I have no idea if it is original. I am getting some fuel smell in the cab (5 year-old stale gas smells wonderful, doesn't it?) There is no indication a charcoal canister was ever mounted anywhere under the hood, there are no vent lines and the fuel pump has only 2 ports.

I know that here in CA, emissions equipment used to be based on the GVW so maybe the 3/4 tons didn't require it? I had a '70 C10 about 10 years ago and I think I remember it did have a canister and a pressure-vacuum cap.

[68gmsee]

[QUOTE=wilbilt;4845398]...I am curious because the cap that is on it appears to be non-vented, although I have no idea if it is original. I am getting some fuel smell in the cab (5 year-old stale gas smells wonderful, doesn't it?) There is no indication a charcoal canister was ever mounted anywhere under the hood, there are no vent lines and the fuel pump has only 2 ports.
[QUOTE]

Couple of things. These trucks were basically made for the specific state requirements. California being one of the strictest. On the other hand, they're also over 40 years old and there have been a lot of people that worked on these trucks and replaced parts including fuel pumps and gas tanks -and not always like the original ones that were in the trucks (I replaced the gas tank on my 69 two times in 25 years).

On my 68, there were microscopic holes on the bottom that would cause a fuel smell. They were so small that I was not able to see any leaks. Basically the gas would evaporating before it puddling.

[wilbilt]

Originally Posted by 68gmsee

Couple of things. These trucks were basically made for the specific state requirements. California being one of the strictest. On the other hand, they're also over 40 years old and there have been a lot of people that worked on these trucks and replaced parts including fuel pumps and gas tanks -and not always like the original ones that were in the trucks (I replaced the gas tank on my 69 two times in 25 years).

On my 68, there were microscopic holes on the bottom that would cause a fuel smell. They were so small that I was not able to see any leaks. Basically the gas would evaporating before it puddling.

Good points. If someone removed the evaporative system, they would probably not have gone to the trouble of removing the vent and return lines from the frame, but you never know.

My gas cap looks just like your photo. The vent does freely admit incoming air, but requires some pressure to vent outward. I suppose I should look for anything on the tank that won't hold 3 PSI or so. Maybe the gasket on the sending unit. The truck has been sitting for years, so undoubtedly some things have dried out.

[harrydunn]

Good stuff guys. Looks like I'm running non-EEC with a vented cap. Seems like people are having trouble sourcing new vented caps that look original.

[cdowns]

i've always just used the stock original caps// took the line that went to charcoal canister ran it back along the framerail to in back of the cab, worked as a great vent without problems for well over 100thousand miles on the trucks i had

[gferris5]

Does this help?

http://67-72chevytrucks.com/vboard/s...d.php?t=479635

(I just made this thread...)

[68gmsee]

The hole is about 3/16th (plus or minus) of an inch and if you insert a small object you can feel the spring loaded vent cap moving. I believe the caps are about 3 psi or so.

[wilbilt]

Originally Posted by gferris5

Does this help?

http://67-72chevytrucks.com/vboard/s...d.php?t=479635

Sure, for that setup you would need a vented cap to prevent pressure buildup which could lead to fuel leakage or engine flooding.

harrydunn, looking through the owner's manual I found in the glove box, it states the EEC was only used on cars and trucks under 6,000 lbs. GVW. It is also noted that EEC requires a "special cap".

From this, I gather that 2500's and 3500's don't have EEC and use a vented cap (for 1971, anyway...)