I believe the cap on a 72 should be non vented,After Shark saw a valve in Corvette Central for 78 to 80, don't know if its the same or if it,will work

I believe it's safe to say that any system with the charcoal cannister has a non-vented cap and tank. That's the way they control evaporative emissions. The car burns the vapor created by evaporation when running and the charcoal absorbs the vapor when not running.

Any idea as to when they started installing the charcoal canister? I have a '69 that seems like it smells most with a fresh tank of gas and the smell isn't coming from under the hood. I've yet to locate any kind of leak or seepage and it almost seems like the smell is coming from the mufflers????

I don't believe I have a canister and I feel that its entirely possible that I'm running on the rich side.

71 for canister and non vented cap, you should have a vented cap and a overflow hose connected to the rubber collar around the fill pipe that runs to the rear bumper.Bad cap, split rubber collar,rotten overflow hose will allow gas to run down the tank and onto the mufflers when a full tank with a bad cap is present. Look for gas stains on mufflers. had that above problem with my 72

anips said: Has anyone had a problem with fuel leaking out of the charcoal canister after filling the gas tank full? Maybe should keep it down from the top a few inches.

The check valve is called the liquid separator valve and depends on the year C3 year determines the configuration. Early years had it mounted at the front of the tank while later years had it mounted on the top. It is a large ball bearing that floats in a cage (late years).

Also in later years to make it work more effectively, a filler gas tube was included in the fuel sender assembly to increase pressure as the tank was filling so the gas pump hose would cut off without overfilling. This became a major problem on C5s and owners were instructed not to overfill...yes C5s. Owners still haven't learned! In the 70's, GM didn't drop owners a note like they do today.

With the non-vented cap in place, the system is pressureized...unless there is a vaccum breach. It sounds like perhaps the problem is several things, overfilling, a breach in the hose system and perhaps a faulty liquid separater valve...though I've never heard of them going bad. If it puked...even overfilling should have prevented this in a normally functioning system...that's my take tho there may be others.

However, I'm not sure what year the originator (anips) of the thread has...in the mid-years (C3) these systems changed more so than in later years.

The canister is the core component in the ECS or Evaporative Control System which captures fuel vapors when engine is at rest from tank and carb, holds them until engine is running and then recycles them for combustion. It operates off engine vaccum and therefore it is critical for effective operation all components be in place, no plugs and no breaks in vaccum continuity. First required on all cars in 1971, California cars came equipped with ECS in previous years.

73shark said: I believe it's safe to say that any system with the charcoal cannister has a non-vented cap and tank. That's the way they control evaporative emissions. The car burns the vapor created by evaporation when running and the charcoal absorbs the vapor when not running.

More emission control minutia:

The C3 model generation experienced the evolution of the 21st century gas cap and it evolved through three phases. In 1968 and 1969 we saw large-diameter gas caps with the word “VENTED” stamped into the cap. In 1970, the large-diameter caps have the words “SEALED” and in 71-72, the words “OPEN SLOWLY CAUTION” stamped onto caps. What we saw between 69 and 70 is the preparation for ECS. Tanks were pressurized to reduce fuel vapors that were once open to the atmosphere and therefore in 1971 contained within the vehicle to meet federal emission requirements.

In 1972, fuel tanks and their containment system were strengthened to manage the higher internal pressures. Tanks, straps and caps were designed to handle these higher pressures. So a 72 cap will work on a 69 Corvette but an early Corvette gas cap is not designed to handle the increased pressures inherent in post 72 Corvettes. Early pressurized caps were designed to sustain 1-2 psi while later caps were capable of handling ??? psi.

The third generation gas cap came in 1975 when the "restrictor" plate was added to prohibit owners from using leaded fuel along with the smaller opening. This is the system that is used in C5 and eventually C6 Corvettes.

BTW...the canister also has a purge valve that permits the saved fuel vapors to be discharged to the carb air intake once engine is operating. This is located at the top of the canister and operates on vaccum.

These systems can be checked by using a vaccum gauge and see if they hold pressure. If the needle on the gauge purges...your system is breached!

Had this happen today on my '72. It's been happening for years if the tank was just filled, driven a few miles, & then stopped. Driving home I said I should check c3vr! Sure enough- 2 pages of good info.

Since all these posts are about 9 months old, did anybody get any definitve answers to their leaks??  My tank has been replaced with new hoses; stock gas cap; evap hoses in engine compartment are new.   Thanks...   

For those of you with a Haynes Corvette Manual, there is a very good description of this (part of ECS System) and basically says a strong smell of fuel is a tipoff that the system is not operating properly.  Some other highlights, a pressure/vacuum gasoline filler cap must be used.  Anything else can render the system ineffective and possibly even collapse the fuel tank.  Says to disconnect the fuel tank line at the Charcoal canistor.  Presence of liquid fuel in this line is an indication that the vent controls or pressure-vacuum relief valve in the gas cap are not functioning properly.  Has other troubleshooting tips also.

Both the fuel tank and the carb float bowl connect to the charcoal cannister.  This is a sealed system.  It's purpose is to prevent gasoline fumes from evaporating into the air.  The fumes from the bowl and the tank both go to the charcoal in the cannister.  This charcoal looks like, and in fact is, about the same stuff you find in fish tank filters.  As the air containing fumes passes throught the charcoal, the charcoal absorbes and stores the gasoline fumes.  It allows air to pass.  This way heat expansion will create pressure, and the cannister can relieve the pressure without haveing the gas fumes in the surrounding air.

When the car is started vacuum is applied to the cannister valve, but the valve is not yet opened.  When the throttle is opened, control vacuum is applied to the control valve, and manifold vacuum is applied to the cannister.  This pulls outside air into and through the charcoal.  This fresh air flow picks up the gas fumes, and dries out the charcoal.  The air and fumes is drawn into the engine and burned.

When the car is shut off, the process starts over.

If the system is plugged between the tank and cannister, the tank cannot vent.  This can create pressure or vacuum in the tank.  Either way is a problem.

If the sytem is clogged or open between the cannister and the engine, it won't purge.  Then the cannister becomes saturated with fuel, and you get a gas oder from the front of the car.  In time, the cannister can be damaged.

If you overfill the tank and the tank gets warm, the fuel can expand and you can get liquid gasoline dripping out of the cannister.

If the tank has any leaks or the lines have any leaks, the fumes won't go to the cannister, and you get gas smells from the rear of the car.

Sometimes the valve assy will fail, and can cause all of the above, depending on the failure.  It can also rupture the diaphram in the valve and suck fume and fuel directly from the tank.  This creates a very rich condition.

The valve assy can be part of the cannister, or remote, depending on the year and engine set up.
All of the cannister equiped cars should have a pressure relief sealed gas cap.  Non canister cars use the vented cap.
kstyer38423.8479166667

What do you guys know about an orifice that is supposed to be located between the tank vapor line and the cannister? Mine i know is missing, is there anything that can be substituted for a replacement? Will not having the orifice in place cause the engine to pull too much vacuum through the cannister?