72 gas leak question

[Gary R.]

Hi Guys
I was out all day today in my 72, stopped at several places no problems then stopped to fill up. I always have filled the tank to the top and let it settle and never noticed anything. Well about 1.5 hours later I went out to drive home and found about a qt of gas leaking out.

I expected to find one of the rubber hoses at the fuel pump leaking but it was under the LH side, behind the front tire. The only thing there is the original vapor can and I clearly saw there was gas dripping out of it. I didn't see any rubber line there though and couldn't get under the car where it was parked.

I made sure there was no issue for a fire and started it up to idle. The drip stopped and I drove home about 15 miles without issue.

I don't have my assembly manual here to look at the plumbing of the tank. I believe there is a black plastic vapor check valve on the tank with a hose running down the LH rail to the charcoal can tucked under the fender. I think I over filled the tank and it ran down to the can but the hose wasn't connected or it popped off?

Does anyone have a picture of this or have experienced this before?

Thanks in advance, I am going to go back out to see if it's leaking again.

Gary

[Bill C.]

Re: 72 gas leak question

Gary -

black plastic vapor check valve on the tank will fall apart with age.
my leak came directly from it.

but I also think - fuel should only run one way. should not pump out of the tank.
maybe the black plastic vapor check valve finally went bad.

I don't think ethanol is very friendly to that part.

Bill

[Russ S.]

Re: 72 gas leak question

I have the same problem with my 73. I replaced the plastic anti siphon valve on the tank and it didn't help. Mine doesn't drip out,it runs out.

[Paul O.]

Re: 72 gas leak question

If you fill the tank to the top and the check float ball in the check valve is stuck in open position. Fuel will flow from the fuel tank to the charcoal emissions canister and leak. This leak will be higher if the car is on a slope with the drivers side at the bottom of the slope. Do not ask how I know this. You must remove the check valve as this is the only way to verify the float ball is stuck. It should rattle when shaken.

[Gary R.]

Re: 72 gas leak question

Hi Guys,
Well this is the first time I experienced this over the 38 years I owned this car. I do recall the check valve was ok when I had the tank out about 10 years ago but like everything else time takes its toll.

I am pretty confident that is what happened, I just over filled it and it ran out the vapor line. The car was parked on a level space too. The last few times I did notice the tank wouldn't fill and stay at a level spot. The level would creep down so it must have been siphoning out past the check valve.

Does anyone know off hand if the valve and filters are still available?

Thank you
Gary

[Alan S.]

Re: 72 gas leak question

Hi Gary,
The fuel separator that mounts on the tank is available from various vendors…Paragon for example. #12439.
The pancake filter for the bottom of the canister is available too, but the canister itself must be decapitated in order to replace the charcoal if needed.
Regards,
Alan

[Russ S.]

Re: 72 gas leak question

Paul, Where is this check valve you are referring to? Is it in the anti siphon or the top of the charcoal canister?

If you fill the tank to the top and the check float ball in the check valve is stuck in open position. Fuel will flow from the fuel tank to the charcoal emissions canister and leak. This leak will be higher if the car is on a slope with the drivers side at the bottom of the slope. Do not ask how I know this. You must remove the check valve as this is the only way to verify the float ball is stuck. It should rattle when shaken.

[Russ S.]

Re: 72 gas leak question

Gary, I have always over filled my 73 from the day it was new, sometimes even bouncing the car to get more in and it had never leaked up until a few years ago. Something had to change.

Hi Guys,
Well this is the first time I experienced this over the 38 years I owned this car. I do recall the check valve was ok when I had the tank out about 10 years ago but like everything else time takes its toll.

I am pretty confident that is what happened, I just over filled it and it ran out the vapor line. The car was parked on a level space too. The last few times I did notice the tank wouldn't fill and stay at a level spot. The level would creep down so it must have been siphoning out past the check valve.

Does anyone know off hand if the valve and filters are still available?

Thank you
Gary

[Alan S.]

Re: 72 gas leak question

Hi Russ,
I believe this is what Paul is referring to.
Regards,
Alan

[Tom L.]

Re: 72 gas leak question

My check valve cracked and the fuel would run onto the muffler below, especially after filling it and making right turns. Scary. I found the leak for sure by just jacking up the right side of the car to get the level up to the valve, fuel started dripping. Not sure about the canister but the valve is available from most vendors. At least it was a few years ago. Good luck!

[Russ S.]

Re: 72 gas leak question

Thanks Alan. As I stated earlier, I put a new anti siphon valve on the tank and still have the problem.

[Gary R.]

Re: 72 gas leak question

Thanks for the pictures Alan. That is how my tank is setup and I recall checking that valve when I had the tank out years ago.

Does anyone know exactly how this works? is there a ball check in there to close when the tank is full but open as the fuel level drops?

[Russ S.]

Re: 72 gas leak question

With valve in the position shown in Alan's first picture, I can blow or suck on both inlet and outlet and get air. If I invert the valve,I can't get air through either of them. I have heard this called a roll over valve which I have always thought to be incorrect however with it closing only in the upside down position, that just might be it's purpose. GM calls it a liquid seperator what ever that is.

[Paul O.]

Re: 72 gas leak question

The check valves main purpose in life is to allow fuel vapors to flow to the charcoal canister. The charcoal traps the vapors and holds them till the engine is started. Then with the engine running air is drawn in to the canister through the filter at it base. The air moves through the charcoal taking the fuel vapors with it to the base of the carburetor and burned during the normal engine operation. The system is the normal gas tank vent.

The check valve in side the body of the separator is designed to only allow vapors to flow along the line to the charcoal canister. If fuel or liquid enters the separator the check ball floats and blocks the fluid from going past the separator. If said ball is stuck and does not float and the fuel level is high enough fuel will flow to the canister and leak out its bottom. This is an early emissions control system.

[Gary R.]

Re: 72 gas leak question

Thanks Paul,
That is how I was figuring it was. I think for now I will be very careful filling up and see how that goes until I decide if I will drop the tank or try and get to the valve in place.

Looking at Alan's picture I am trying to visualize where the level line would be in relation to the valve hose fitting and fill tube?