Full Tank Fuel Pressure on a Hot Day... Fuel everywhere.

[Spool]

Hey All.

73' C20 here.

Clearly the gas expanded on a hot day with a full tank, and fuel went everywhere.
Question is. How do I stop this. Is it not ventilated properly? Do I need to drop the tank? Is there an easy fix, like a vented gas cap?

Temps are going up here in Los Angeles and I was driving around a lot yesterday. So I topped off my tank before hitting the highway for 11 miles.
Well once I got there, I noticed fuel escaping from my gas cap. I take the cap off to release the pressure and to my surprise, 3 gallons come rushing out instead of the expected
few ounces.

Thanks in advance.

[VileZambonie]

So I would advise you to properly vent the tank. In the interim you can vent the gas cap until you correct it just don't walk by with a lit cigarette and don't overfill it.

[Spool]

How do you properly vent your gas tank?
I would have assumed this wouldn't be a problem, if manufactured right. I can't imagine anybody messing with it over the years.

Do I need to drop the tank?

In short it's really confusing as I've had the truck two years and it's never been a problem. Yesterday was the hottest day this year, but still, it was only 73degrees.
My full tank has seen hotter days.

[Dr_Snooz]

The tank should be vented through your charcoal canister. Your evaporative emissions control system is clogged.

[Spool]

Makes sense. I found this page on a quick search.
http://www.aa1car.com/library/evap_system.htm

Does anyone know if this is an easy/difficult or inexpensive/pricey job?

[Peter S]

The tank should be vented through your charcoal canister. Your evaporative emissions control system is clogged.

His truck truck is a 1973, the charcoal canister didn't show up until 1979

[VileZambonie]

The tank should be vented through your charcoal canister. Your evaporative emissions control system is clogged.

His truck truck is a 1973, the charcoal canister didn't show up until 1979

I think you've been misinformed

[Rapid Roy]

("His truck truck is a 1973, the charcoal canister didn't show up until 1979")


My 74 has a canister and also the 72 I use to have.

[Peter S]

okay, I am misinformed. My 78 doesn't have one so the previous owner of my truck must've changed the set-up.

[Dr_Snooz]

The evap system is about as simple as can be. There is a charcoal canister and some rubber tubes going from the canister to the tank and intake. The canister absorbs gas odors when the engine is stopped. When the engine is started, a valve allows vacuum to draw the fumes out of the canister and into the engine to be burned. If your truck stinks up the garage with fresh gas smell, that indicates a problem with the evap system. Things rarely go wrong, but occasionally the canister plugs up and needs to be replaced.

[VileZambonie]

okay, I am misinformed. My 78 doesn't have one so the previous owner of my truck must've changed the set-up.

Back in the day the first thing you did was rip all of the emissions controls out and most of the aftermarket carburetors were not setup for plug n play action with charcoal canisters, so everyone ripped them out and tossed them. For a prettier engine compartment you can easily hide the charcoal canister underneath the truck. If it's been removed and lines plugged, you can take apart the gas cap and make a small vent hole behind the cap cover but that is a band aid fix. It's still better than having an unwanted fuel eruption though.

[hatzie]

The funny thing is the charcoal can is so bloody simple a caveman could hook it up and there's no performance hit at all.  The very early three hose cans are darn cheap and can be placed next to the battery tray nearly out of sight.
The purge hose was usually teed into the PCV hose to manifold vacuum and the purge valve vacuum actuator can be hooked into one of several ported vacuum ports that were available for distributor vacuum advance on even the cheeziest aftermarket carbs. 

[Spool]

Well, I'm a bit embarrassed and primitive I suppose as this is all news to me.

I'll have to research this when I have time off this week and figure out what to do to get it done. For example locating it in the first place.

Anything in general a newbie should know before attempting to swap a charcoal canister?
Am I to expect any fuel spillage when disconnecting any lines?

Thanks for all the responses guys. Helps a lot.

[blazer74]

 No spillage you will be fine.

[hatzie]

No spillage you will be fine.

It's just a vapor line from the tank there shouldn't be any liquid in that line.

Purge will be the largest line.  Tee that into the PCV hose.
Purge valve actuation vacuum can tee into the distributor advance line.

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