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Thread: Fuel Tank Overlows When Filling Up

  1. #1

    Fuel Tank Overlows When Filling Up

    Hi all,

    When I purchased my 1970 E, the fuel tank ventilation system had been completely removed. I’ve now restored the system to what I believe is the original configuration. I’ve now filled the tank up twice and both times, the gas starts to overflow into the overflow tank before the pump will cut off. Each time upon fill-up, I ‘ve opened the hood to watch the fuel tank ventilation hoses and each time the gas spills over into the 3/4" hose next to the sending unit. Is this normal? I would have thought that pump would cut off once filler neck filled up but I don’t see how this can happen since the filler neck is much higher than the overflow.

    Thanks

    Chris

  2. #2
    Senior Member
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    It seems like the system is operating correctly. If you look at the design, the purpose of the tank (2) in the left fender well is to hold a small amount of overflow. As the fuel level goes down the overflow empties back into the fuel tank. Fuel fumes are directed through the charcoal canister back to the intake manifold to be burned instead of being released into the outside air.
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    E Sully
    1973.5 911T

  3. #3
    Senior Member
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    Something is not correct , there is no overflow tank , it is an evaporation tank .

  4. #4
    Member #226 R Gruppe Life Member #147
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    I’m with Richy, something’s wrong.

  5. #5
    Senior Member Nicklasliljegren's Avatar
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    Me 2
    I dont have this problem with my 1970 911 E US-spec

    Best Nick

  6. #6
    I just checked the shop manual and they do call it a "vent chamber". I was incorrect to call it an overflow tank. So when you fill up, gas should not typically spill over into the vent chamber correct? Does anyone have pictures of how theirs is setup? Here's how I set mine up.

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    Chris

  7. #7
    Although the photos don't display every hose connection, what is shown looks correct....but originally each of the hoses had band clamps at the plastic fittings. Another thought...have you by chance filled up at the same pump each time you've had this 'overfill' happen? If so, you might try another station/pump before you make yourself crazy. I've never had what your experiencing on my 70, 72 or 73 cars.

  8. #8
    Member AKAMick's Avatar
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    Quote Originally Posted by mac73s View Post
    but originally each of the hoses had band clamps at the plastic fittings.
    when did they install band clamps at the plastic fittings as my 70T that i have known from it's delivery never had them?,excuse the Ziebart goo that was applied one week after it's delivery,Name:  100_2510.jpg
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    Current fleet, mostly rubbish automobiles and other assorted junk.

  9. #9
    Mac73s - You make a good point. I've filled up at the same station each time. What I can't seem to wrap my head around is that the vent line coming out of the tank is much lower than the top of the filler neck. If the pump where to cut off automatically, gas would have to be in the filler neck thus causing the gas to spill over into the expansion tank. I don't see what would stop this unless I have some of the vent piping located to low.

    Chris

  10. #10
    Senior Member NorthernThrux's Avatar
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    Chris, Your piping looks correct. On some occasions, I too get gas in the fender mounted tank. It eventually evaporates and gets captured in the system. My garage always has this faint waft of gas for the first day or so after a full fill. This seems to be a function of the particular gas station nozzle. I find that most nozzles cut out aggressively as I am filling in my 911. If I modulate the fill rate, I can fill into the neck of the tank and that's when I run into this issue. Nozzles are designed with a venturi effect sensor which is heavily influenced by foam and froth at a little hole right at the bottom tip of the nozzle. Normally that little hole is sucking in air, but as soon as froth or liquid interfere with it, it triggers the shutoff. But this is highly variable in my experience. Many are calibrated for the rubber cutoff that seals the filler hole as you are filling (preventing evaporative emissions), but others are not. The vapour in the fender tank should waft through the charcoal canister. Depending on age and effectiveness of that canister, that might not fully do it.

    Also note that if fuel is in those Wye connectors, it rots them pretty fast. They aren't meant to be immersed in fuel, only vapour. The vent chamber can indeed collect overflowed gas at any rate. It will either drain back in time or evaporate as noted.

    Ravi
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

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