rogerstopford

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

thomasmryan

you might pull the canister from the front drivers fender well and shake it to see if the carbon pellets have escaped into the vacuum system.

you may have a faulty 'roll over' valve

rogerstopford

Thanks, we are going after the carbon canister now. We figure the most likely thing is that the gas is coming from the vent pipe from the carbon canister, due to some problem in the out line from the carbon canister which is then pushing gas up the vent pipe from the carbon canister. We renewed the vacuum valve a while back, so that just leaves the bleeder valve, and the moulded hoses. Thanks for the idea about the carbon pellets blocking the vacuum system.

rogerstopford

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?

thomasmryan

The purge valve is located below the control dashpot by the brake booster. With the engine warmed, you should feel it pulse on engine overrun. The DME waits until the engine is up to temp to start scavenging from the charcoal.
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rogerstopford

Thank you for the help but let me just try to be sure which parts you are referring to. When you say purge valve, is that what Porsche calls the #14 bleeder valve? And the brake booster is the big thing by the brake fluid reservoir, so I am in the engine bay looking for the bleeder valve below the brake booster, but what is the control dashpot?
And thanks for the operating info - yes, I was figuring the #15 vacuum valve draws vacuum from the intake system and opens the path from the carbon canister output to the engine fuel input somehow, taking excess fuel into the engine, but only when the engine is ready - for instance warmed up.

thomasmryan

Follow the larger vacuum line up from the intake near the drivers side motor mount arm. The purge valve has an electrical plug similar to the fuel injector connection. The next valve is controlled by manifold vacuum.
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