Gas Tank venting

[car911]

Does anyone know how the 911 gas tanks are vented. I know about the carbon cannister. I need to know how air is vented INTO the tank as fuel is used.

As result of emissions inspection, I had to replace my gas cap. Unfortunately, the new cap seals the tank too well, so I get a strong vacuum as the tank empties. I think this is buckling the tank slightly, causing problems with my fuel guage sensor.

Is there a diagram showing how the tank is vented?

[singpilot]

Did you keep the old cap?

Emissions vs. Collapsed tank..... hmmmmmm.

[HarryD]

Here's a sample of how they did it in 1970:

[BGCarrera32]

Originally posted by car911
Unfortunately, the new cap seals the tank too well, so I get a strong vacuum as the tank empties. I think this is buckling the tank slightly, causing problems with my fuel guage sensor.

No offense intended but I seriously doubt it. You'd need a heckuva lot of vacuum to collapse your steel tank to the point where it would foul your sensor. If you somehow pulled that much vacuum at the tank the rubber lines would collapse first. The most likely culprit is the sensor itself. What kind of problem are you having with it, and what year car?

-BG

[singpilot]

In college, I collapsed a gas tank in a Merc Marauder the first week that I-5 was open south of Modesto. As tank collapsed, it captured the sender at 1/3 reading. I couldn't believe the mileage I was getting until in the dark, it ran out of gas. 25 miles from anywhere, pre-cell phone era. My GF and I (totally in the darkness, not a light anywhere on the horizon) waitied by the car until someone came by. Took several hours to get a ride back with 5 gals of gas. Tank filled after 3 gallons. Non-vented cap had let fuel pump suck tank flat. GF insisted I did this to get her marooned for a make out session....... Ran out of gas, right.

[car911]

Originally posted by BGCarrera32
No offense intended but I seriously doubt it. You'd need a heckuva lot of vacuum to collapse your steel tank to the point where it would foul your sensor. If you somehow pulled that much vacuum at the tank the rubber lines would collapse first. The most likely culprit is the sensor itself. What kind of problem are you having with it, and what year car?

-BG

No offence taken at all. I find it hard to believe as well. However, it only takes 1/4 inch or so to crunch the fuel sensor which is made from 'beer can' guage aluminum. And the Porsche gas tank is comparatively large with large sheet metal surfaces which can, I think, easily buckle a fraction of an inch especially between the top and bottom surfaces.

When I open the gas cap to fill the tank, there is a very strong air intake. I recently put in a new sensor and the mechanic had to actually press the tank bottom out a bit with a rod to fit the sensor. I am on my second new sensor now.

I have also heard from automotive engineers that if and when the carbon cannisters and venting lines clog up over the years, it is not uncommon for tanks from american tanks to collapse a few millimeters from the vacuum.

(I also foolishly threw away the old cap!).

[car911]

Originally posted by singpilot
In college, I collapsed a gas tank in a Merc Marauder the first week that I-5 was open south of Modesto. As tank collapsed, it captured the sender at 1/3 reading. I couldn't believe the mileage I was getting until in the dark, it ran out of gas. 25 miles from anywhere, pre-cell phone era. My GF and I (totally in the darkness, not a light anywhere on the horizon) waitied by the car until someone came by. Took several hours to get a ride back with 5 gals of gas. Tank filled after 3 gallons. Non-vented cap had let fuel pump suck tank flat. GF insisted I did this to get her marooned for a make out session....... Ran out of gas, right.

This is exactly what happened to me. Car died in the middle of an highway on-ramp with the gas guage at half-full. Had it towed to the mechanic. He spent a half a day on it (injectors, fuel pump, spark, etc, etc) before figuring it out. Both of us feeling pretty foolish. (Never thought to mistrust the gas guage!) Replaced the sensor. Happened again. This time, we checked carefully and discovered the sensor was crushed by about 1/8 - 1/4 inch. Pushed out the tank bottom a bit and replaced the sensor again. I think the second sensor is being crushed again. Each time I open the gas cap, there is a tremendous 'woosh' as air gets sucked into the filler tube.

Will replacing the carbon cannister relieve some pressure? Or are there other venting lines I should check?

[singpilot]

I think I would find a vented gas cap until you get the vent system checked out.

[Jim Sims]

Although it seems counter intuitive it really takes very little vacuum to collapse and buckle thin wall vessels especially of construction and aspect ratio typical of automotive fuel tanks. Without venting, a fuel pump or even thermal contraction of fuel and fuel vapors if the tank is sealed while hot and then cools significantly can produce enough vacuum to cause problems. Jim Sims

[Chuck Moreland]

It's not factory, but here is my solution:

Need to vent, my gas tank

This has been working well.

[thrown_hammer]

Not to hijack but here is a question...
I have a 1973 with a 3.2 in it. I dont have the front charcoal canister anymore. The lines are disconnected on both ends. I do have the fender expansion tank. Do I need it with the 3.2???

[singpilot]

Being in both P-car worlds (water cooled and oil cooled) I could really use an air cooled Porsche clock.

(Word Count = 19; P-car content checked, Word "PORSCHE" found, line 1)

[juanbenae]

did they keep the original porsche (500k post req) cap? if not why did'nt you drive around the corner and replace it?

let see, 10 words,,,, check

[surflvr911sc]

You can get new a seal for the cap and it will pass smog w/ no problem, of course, you threw it away. Did you get a new factory one?

[car911]

Originally posted by surflvr911sc
You can get new a seal for the cap and it will pass smog w/ no problem, of course, you threw it away. Did you get a new factory one?

I threw away the old cap (it didn't occur to me that there would be a problem). I got the new cap from the local Porsche parts distributor. Does anyone know if all replacement gas caps are the same?

[singpilot]

I just replaced the gas cap on the 944. It has a feature where you can turn it a half turn, and it releases the pressure/vacuum. I think I got it at NAPA.

[sammyg2]

Check your cap, it probably has a large rubber o-ring on it. For the short term you could carefully remove the o-ring, voila, self venting cap.

[singpilot]

Sammy! That's thinking outside the box! Us '80 911SC guys are known for that!