I have a 968 coupe that has a weird issue, for about 1 year. Every now and then, with a full tank, warm weather, after some regular driving, then park, the car will dump gas onto the ground, around the back of the passenger tire. Not much smell of gas inside the car.
And the gas tank has huge negative pressure - big sucking of air when you open the gas cap.
So makes no sense, right? Negative pressure in the gas tank, but pushing gas out of somewhere.
Searched forums, looked at the diagrams and it seems there is a lot of valves and stuff in and around managing fuel pressure.
The ball bearing in the shutoff valve seems ok.
Have dismantled the entire interior around the filler neck - no sign of leaks or cracks.
Not leaking around the fuel level sensor assembly - on the top of the fuel tank.
Not leaking from the expansion tank.
The pipe from the expansion tank to the shutoff valve smells of gas - but it should, so thats not much of a clue.
Any ideas?
Thanks

 

It's not sucking air, it's blowing it.

 

yes, that's what all the info says, but it sure seems like its sucking. I will check, if I can provoke it to happen.

 

There must be a line blockage somewhere. Maybe the line going to the charcoal canister? Also check out the return line from the engine back to the tank, it's steel..you might have a pinhole or the tank is overflowing from the heat, especially with a full tank.

 

Thanks, yes, we have been studying this page of the manual: We figure the fuel from the expansion tank is getting blocked after the carbon canister, and so is backing up through the carbon canister vent line. Does anyone know where the carbon canister vent line actually ends up? Also, on the picture, does anyone know where the output pipe from the bleeder valve goes? Is this what Tiger03447 is referring to as "the return line from the engine back to the tank"?

 

Not sure about the 968, but on the 944 Turbo, the #8 vent line is below the drivers (left) floorboard cable tied to the plastic cover.

 

Hmmm, well if it stops there on the 968 that cannot be where the gas is exiting, because it drips by the passenger rear wheel

 

If memory serves me correctly the mystery line termination you are referring to goes to the intake manifold to allow the gas fumes trapped in the charcoal canister to be purged into the engine and burned. Someone pipe in and second this or correct me if I'm wrong.

 

That's generally correct. He may also have a stuck purge valve between the canister and the intake manifold. This valve is usually controlled by a signal from the computer to open when the car is started . Interesting that the puddle is by the RR tire. Is there a possibility that the gas is leaking into the RR cubby and then draining out? Maybe a cracked hose that only opens up under pressure? He does state that he has checked all things in the filler neck...however. Sounds like it's time to pressurize the tank with compressed air through the filler port to get a good look as to what is happening...soapy water on all the connections. Good luck!

 

I had an issue with the line between the fuel tank and the pump - it disintegrated from the inside out and separated around the internal wire mesh. A small crack developed in the outer layer and it would drip fuel after being left for a time. When the car ran, the vacuum "fixed" the issue by sucking the outside of the separated line against the inside, forming a seal.. The drip was worse with a fuller tank, likely due to the weight of the fuel pressing the line apart. Eventually replaced the line, (and the pump and filter to ensure they weren't full of rubber chunks.)

 

Looking up inside the passenger side wheel by the gas tank, there are surfaces wet with gas when it is happening (full tank, been driving, warm weather). Right by/above those wet areas is this pipe outlet. Anyone know what outlet this is? Is it possible it is the vent from the carbon canister? Hard to see it, but easier on the second pic. The first pic below is this area on the 968 with the problem. The second pic is another 968 without the problem. Also, I figure it's something to do with venting (big whoosh when opening gas cap) so occurring due to pressure, but I cannot work out how the venting works. The Porsche-called shut off valve is between the expansion tank and the carbon canister. Then there is a Porsche-called bleeder valve between the carbon canister and the not-sure-where-it-goes-but-I-figure-intake-manifold. The flow between carbon canister and bleeder valve looks like it is controlled by a vacuum valve, which looks like it is controlled by the engine computer.
We are currently dismantling to access the carbon canister.....

 

Are you possibly over-filling? I fill until the auto-shut off at the pump stops the flow and sometimes get a fuel smell (no leak in this area though); I figure the auto-shut offs are a little inconsistent from gas station to gas station.

The gas cap isn't vented, so maybe something is wrong in the lines. What's #26 in the diagram? Could that be malfunctioning? BTW #8 on a 951 just goes to the outside under the car (yours routes back to the gas tank). My understanding is that #8 isn't a vent out but a "vent in", meaning a line that allows for fumes to flow to intake when "bleeder valve" and "vacuum valve" allow them.

Perhaps you can just disconnect at #24 and see if the issue disappears?

 

You might try disconnecting the vent line to the charcoal canister and see if it's letting air through. If it's full of gas, then you might have a bad breather valve. After disconnecting, remove the breather valve and check it for flow..then disconnect the line at the canister and pressurize it. There should not be any air escaping from it. If you can rig a gauge on the canister end, then you can check for a pressure drop in the line which will tell you it's a line problem and not a tank problem.

 

Hi Dan, sorry, #26 is called "shut off valve". Thanks for the info re the vent-out vs vent-in. The whole system is hard to understand.

 

Hi Tiger03447, when you say 'vent line' do you mean #8 in the Porsche picture?