Somewhere along the line I lost my gas cap - so I replaced it with a locking cap which turned out to be non-vented (my mistake). All was well until yesterday when I filled the tank up and hit the freeway. (Since the car has just come out of a major restification it has only been driven on short trips and I had not filled the tank up all the way recently)

Anyway, once I got stopped at my destination a big puddle of gas collected under the rear. Turns out that a small leak magically appeared at the base of the fill pipe at the tank joint. The non-vented cap apparently allowed pressure to build up inside and caused the gas to make a fine little pee stream out the new hole and spew all over.

Switching to a vented cap and squishing some tank repair goo over the hole should fix the problem (temporarily) but here's the real question.....

Why is there pressure built up in the tank in the first place? Logically, it seems that if gas is being drawn out of a sealed tank it should create a vacumn and eventually stop the flow of fuel. (This will naturally happen as I pull into the local cruise spot and my car will die right in the driveway!) Instead there is a pressure build up - is this just gas getting agitated by the drive and then vaporizing?

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

I suspect it was gas vapors since the gas tank is next to the exhaust pipes. I bet the pressure of the gas vapor will overcome the vaccum created by drawing fuel out.

No exhaust effect - the pipes end in front of the rear end until I get my new exhaust system.

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

If it's a 69, I think there's a vent line from the fuel pump to the tank, that might account for it.

No vent line - there was one originally but the tank guage was replaced by a single line unit so there is a single gas line - no return.

I patched the tank leak with some tank repair epoxy and changed the cap to a venting style and so far all is well. The real test will probably come when I fill it up again and run it for a longer time period.

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

Scott,
Are you sure the filler neck is cracked at the joint? My car had a similar problem, but the repair shop presurized the tank and said it was OK. Turned out to be a faulty gasket in the cap. This only applies if you parked on an incline. Fuel evaporates easily due to a low boiling point this is why it builds pressure in the tank, and is also why a near empty tank is a greater exlosive hazard than a full one.

snakedr - I was parked on the flat and there was an obvious little stream "peeing" out of the filler neck seam where it attaches to the tank. It continued to leak from gravity alone after the cap was removed and pressure released. I jacked the car up so gas was no longer covering the seam and dried it out and cleaned it with brake cleaner and 409. Then I squished some epoxy over the area and let it harden. So far so good and no more leak.

I think the extra pressure from the non-vented cap "found" a weak point in the seam - it sure wasn't doing it before!

BTW - I see from your profile that you deal with aircraft recovery - sounds like the old Douglas Aircraft Co. perhaps?

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)

Rocky Mtn...
I live in Colorado Springs...real close to you so one thing I have noticed. The gas comes out of the tanks fairly chilly i.e. aroung 40-45 degrees F. This is because the underground tanks keep a pretty constant temperature in this range. As you know, things expand when heated, so I have seen gas cans that when filled full and then placed in a warmer place, will build up pressure just as you are describing. I think this could potentually cause your problem if the seam was weak to begin with. Maybe this summer we can get together and compare notes or something...

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Scott you are absolutely correct. DAC now employed by the Lazy B.

Gasoline has something called a Reed Vapor Pressure. That's the amount of pressure produced by the fuel vapor evaporating into the open space inside the fuel tank, at a given standard test temperature, I assume.
Once this pressure is acheved, the fuel stops vaporizing. I believe it's close to six pounds at 70 degrees. Newer fuels here in California have eather in them which has quite a bit of vapor pressure. That is why vapor recovery systems have been a high priority with the government. They even have check valves on the storage tanks.

Thanks for all the comments! It's been 5 days now since I applied the patch and all is well. For all the reasons provided there was expansion in the tank and no relief so the fuel found an outlet. Anyway - it's fixed for the time being - although I don't like the idea of a patch so I'll keep an eye on it and maybe pick up another tank when the opportunity presents itself.

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Scott
'69 400SB, Richmond 5-speed; '99 HD Road King Classic
www.geocities.com/sdenning1 (http://www.geocities.com/sdenning1)