1973 leaking fuel through charcoal canister

[Russ S.]

Can any one tell me what is causing fuel to run out of the charcoal canister on my 73 when the tank is full? I have replaced the fuel separator.

[Alan S.]

Re: 1973 leaking fuel through charcoal canister

Hi Russ,
ANY chance that the purge valve on the top of the canister is malfunctioning and the engine vacuum is pulling gas from the tank when it's at a certain level?
REALLY reaching here!!!
Regards,
Alan

[Russ S.]

Re: 1973 leaking fuel through charcoal canister

I suppose that is a possibility Alan. I will check it. I was reading other posts which say over filling the tank can cause the leak but I filled it the same way I have for 40 years. I do fill it above first pump kick off.

[Duke W.]

Re: 1973 leaking fuel through charcoal canister

Back in the day I would fill my SWC to the point where the fuel was literally halfway up the filler neck. That's a no-no on a car equipped with an evaporative emission control system. There must be a "vapor dome" at the top. Otherwise if the fuel gets hot and expands, liquid fuel rather than vapor will be pushed into the cannister.

Most cars with evap controls have a built in "vapor dome" at the top of the tank, which is usually accomplished by placing the top of the fill neck slightly below the top of the tank, but the basic Corvette fuel tank design from 1963 prevents this.

Bottom line, don't fill it even to the bottom of the fuel neck unless you're going to drive it at least ten miles or so to burn off at least a half gallon.

It's easy to check the purge valve function with a Mity Vac or equivalent.

Duke

[Edward J.]

Re: 1973 leaking fuel through charcoal canister

Can any one tell me what is causing fuel to run out of the charcoal canister on my 73 when the tank is full? I have replaced the fuel separator.

Russ, I have pulled the vacuum diaphragm on the canisters before if you want to check the diaphragm the cap on top will pop off and the is a spring and diaphragm under the cap. Over filling and expansion as Duke says will make its way to canister.

[Alan S.]

Re: 1973 leaking fuel through charcoal canister

Hi Russ,
In looking through the GM Emissions Control Manual for 71 I see a section about "Fill Limiting Systems" that illustrates exactly what Duke described.
In it the tanks are shown either with a 'dome' that works in controlling air bleed off, or show limits on filling.
It appears that by 71, (and in Corvette tanks,) the instructions are to fill the tank no further than to limit the amount of trapped air in the tank to the volume of 2 gallons of air.
There is an explanation of the operation of the purge valve, but not a test procedure for it. It does indicate though that if the pancake filter at the bottom of the canister should become saturated the engine will be forced to pull air from the tank rather than through the bottom of the canister.
Hope an answer for your situation is here somewhere.
Regards,
Alan

[Duke W.]

Re: 1973 leaking fuel through charcoal canister

The operation of the evap cannister is pretty simple. There are four hoses - carb bowl, tank, signal, and purge.

The signal is from the carburetor venturi and at a certain vacuum level it will open the purge valve and manifold vacuum will draw air through the filter at the base of the cannister and a restrictor to purge the charcoal of absorbed hydrocarbons.

So use a Mity-Vac to pump down the signal line and the purge valve should open up at probably no more than 10-12" Hg vacuum.

There may be more information in later CSMs. Evaporative emission controls were first required in CA in 1971 and 1973 in the other 49 states.

Duike

[Russ S.]

Re: 1973 leaking fuel through charcoal canister

Thank you fellas for the great info.

[Alan S.]

Re: 1973 leaking fuel through charcoal canister

Hi Russ,
Looking at the location of the inlet to the separator makes it appear that over filling the tank would allow fuel to enter the separator.
But the outlet from the separator appears to be placed higher than the top of the tank so fuel in the separator would drain back into the tank unless the engine vacuum was pulling it through the canister.
Purge valve or pancake filter?
Regards,
Alan

[Terry M.]

Re: 1973 leaking fuel through charcoal canister

Guys,
The fuel filler neck for C3s extends down into the tank about 1/2 to 3/4-inch. This allows for the "dome" of air Duke talks about -- unless one modifies the fuel filler neck. Don't believe me? Stick your finger under the edge of the filler neck and see how far up the inside of the tank you can get it. Don't let anyone see you doing this. In most communities you can get arrested for deviant behavior doing this.

I wouldn't be surprised the later C2s are constructed the same way, but then maybe not. I have no intimate knowledge of C2s.

[Rich C.]

Re: 1973 leaking fuel through charcoal canister

I wonder if that separator is defective, I was under the impression there was a check ball/valve in it to prevent that very occurrence. I wouldn't imagine the charcoal is going to be very effective if it's saturated with fuel. I was told a long time ago that the charcoal shouldn't 'rattle' in the canister if you shake it. I have two brand new canisters, one N.O.S. and one service replacement, neither rattle, but my original did. Not entirely related to your problem, but maybe noteworthy?


1973 LS-4 454 coupe owned 24 years
1996 LT-4 Collector Edition coupe owned 13 months
Sierra Nevada Torpedo IPA-currently on tap!

[Russ S.]

Re: 1973 leaking fuel through charcoal canister

Hi Russ,
Looking at the location of the inlet to the separator makes it appear that over filling the tank would allow fuel to enter the separator.
But the outlet from the separator appears to be placed higher than the top of the tank so fuel in the separator would drain back into the tank unless the engine vacuum was pulling it through the canister.
Purge valve or pancake filter?
Regards,
Alan

Or if the full tank was being pressurized due to outside ambient temp???

[Russ S.]

Re: 1973 leaking fuel through charcoal canister

I checked my original fuel separator which I had replaced earlier due to this fuel spilling problem,(didn't help) and it either has no valve in it or it isn't functioning. If I shake it tho there is something inside there moving around. Maybe a float valve???

[Steve L.]

Re: 1973 leaking fuel through charcoal canister

i always had problems with the separator since new. If I filled that tank and parked on a hill with the back end up, fuel will come out of the canister. The fix was not parking on a hill. I never went to the dealer for this but maybe I should have.

I replaced my old original with a new one during restoration so I will see if that fixes it but I suspect not. Both new and old rattled like there is a ball check valve inside. I never took one apart but I suspect that the design of this is poor.

steve L
73 coupe since new

[Russ S.]

Re: 1973 leaking fuel through charcoal canister

i always had problems with the separator since new. If I filled that tank and parked on a hill with the back end up, fuel will come out of the canister. The fix was not parking on a hill. I never went to the dealer for this but maybe I should have.

I replaced my old original with a new one during restoration so I will see if that fixes it but I suspect not. Both new and old rattled like there is a ball check valve inside. I never took one apart but I suspect that the design of this is poor.

steve L
73 coupe since new

Steve, That is really good to know. What I can't figure is why didn't I run into this problem all these years since I bought it new in 73 until the last couple of years. I have always filled the up as full as I could easily get it.