how low can you go... fuel tank capacity

Discussion in 'Fiesta ST Chat and Discussion' started by ertzog, Mar 23, 2014.

  1. ertzog

    ertzog New Member

    managed to go 14 miles past "zero miles to empty" and then put in 12.5 gallons.

    what is the most anyone has put in without going empty / after going empty?

    wasn't trying to.. just turned out that way.
    LotusZX3 likes this.
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  3. D1JL

    D1JL Well-Known Member

    I try very hard not to do this as the fuel pump is cooled by the gas in the tank.
    Continually running the tank very low or empty will reduce the life span of the pump.

    That being said, it is good to know the capabilities of your car.
    Just don't make it a common practice.

    Sekred likes this.
  4. ertzog

    ertzog New Member

    ford lists the fuel tank as 12.4 gallons.... that and some inlet piping suggests it was pretty close to middle of the night dead on the interstate (nebraska).

    what was nerve rackingly unknown, was how closely ford calibrates the "miles to empty" on the dash display.
  5. jimclark

    jimclark Active Member

    Dunno, and don't wanna find out. I try to keep at least a 1/4 tank at all times, and prefer to stay on the north side of half-full. I also keep a go-bag in the car, some flares, and an axe. Just in case of the zombie apocalypse.
    djdennehy and spangenb like this.
  6. Sekred

    Sekred Active Member

  7. bigshotdan

    bigshotdan Active Member

    I've not had the ST too close to 0km DTE yet, but used to regularly do it when I had the RS, because Ford didn't recalibrate things for the larger fuel tank in that model. Don't think I'd be game to chance it in the Fiesta, as I don't think its tank is bigger than the rest of the range...

    On another note, that was an eye-opening read about condensation. Never encountered that one before...
  8. wash

    wash Active Member

    The condensation argument doesn't sound right to me.

    If you always keep your tank full, lots of junk from the fuel can settle in the sump because its never getting close to empty, then the first time you have to drive until its near empty, that's when all of the garbage hits your fuel filter all at once.

    I guess we don't hear about water in gas because almost everyone has an ethanol blend and for Californians, we don't have a lot of humidity.

    With an Ethanol blend, if it can absorb water, going until near empty is going to remove most of the ~contaminated gas and then the fill up dilutes it to a great extent. If you never run close to empty, the dilution is not much.

    For arguments sake, let's say 100% humidity is the same as straight water vapor which I think would weigh roughly 36 grams for 22.4 liters and our gas tank holds about 50 liters. Let's say we run it down to 5 liters gas and 45 liters of water vapor. That's about 72 grams of water vapor maximum or 72 cc's volume if it all condenses but you still have 5,000 cc's of gas. Worst case you're roughly 98.5% gasoline, 1.5% water. Real world its probably more like 0.15% because the tank would never be full of water vapor, then you fill it up with another ~45 liters of gas and the dilution takes the water concentration to 0.015%.

    In regular conditions, I wouldn't worry about condensation unless regular conditions means driving 1 mile every day with big temperature swings to form condensation and fill ups so infrequent that water can accumulate.

    In that case, I would want to use fuel stabilizers more than alcohol to absorb water or keeping the tank full.
  9. Sekred

    Sekred Active Member

    Where do you think the fuel pump picks the fuel up from?, the bottom of the tank. You really need to think before you open your month wash. Some of the stuff you come out with has real merit, other stuff like that is just complete rubbish. As for trying to work out how much condensation is going to form in the fuel tank, lmao.
    Smokin likes this.
  10. wash

    wash Active Member

    That's funny, why do you think drag racers put extended sumps in their gas tanks?

    Stock tanks pick up near the bottom, not the bottom. Manufacturers are funny about not wanting anything sticking out under the tank that could get torn off in a crash, especially Ford (ever heard of a Pinto?).

    My condensation estimate is a worst possible condition just to find out what ballpark we are in.

    So where are the conservative and scientific figures showing how keeping your tank ~full reduces water contamination? I didn't see anything accept a note that surface area is bad.

    Give me a citation or show your work, they did neither.

    Let's see your math.

    For all I know, I could be wrong or you could just be calling me names.

    I'm not afraid to be wrong, I just won't take your word for it.

    I await a rebuttal and I may even look up a better estimate of how much water can condense out of 45 liters of air at 100% humidity instead of just guessing based upon my observation of fog and the density of O2, N2, CO2, etc.
  11. Sekred

    Sekred Active Member

    Mate, I think you may be just looking for a argument, simple as that. What you said and I will quote " If you always keep your tank full, lots of junk from the fuel can settle in the sump because its never getting close to empty, then the first time you have to drive until its near empty, that's when all of the garbage hits your fuel filter all at once ".
    Thats incorrect and illogical . Don't you think that when you drive around and the fuel moves in the tank that the contamination will mix with the fuel and then get sucked up by the pump pickup whether its full, half full or near empty?
    As far the location of the pump pickup being not directly at the bottom, fair enough but it is very close to the bottom for obvious reasons.
    Ford Pinto, I've read the story before. Can't see the relevance.
    As for the maths, I have no interest in calculating exactly how much condensation may form. How could it possibly be accurate?, and of what use?.
    Fact, under the right conditions condensation forms in fuel tanks. The lower the fuel level the more condensation that can form. If you want to calculated it, go for it.
    Is it enough to cause problems?, well in common rail diesel engines it can cause injector failure where the tips are blown off and damage to the high pressure pump. Gasoline injection?, runs significantly lower pressures but I would prefer not to find out.
    Once water enters fuel it forms in 3 ways, Free state, Emulsified and Semiabsorbed. Its the semiabsorbed that can do the real damage to the fuel system.
    What pissed me off was the fact you simply dismissed the idea that condensation could be create real problems when you have No first hand experience with the issue. Google will only get you so far.
    There was no name calling, I gave a rather harsh option of the first part of your post and laughed at the idea of trying to calculate the amount of condensation that may form in certain conditions ,a little unkind so I apologise for the Lmao part, but I have seen first hand the damage water in fuel can do.
    Smokin likes this.
  12. LotusZX3

    LotusZX3 Member

    Thanks for the heads-up there. I haven't got into a sticky situation yet but now i know i have a little wiggle room after the MTE hits zero.

    I typically fill up when the light goes on or before.
  13. wash

    wash Active Member

    How many old gas tanks have you looked in?

    I've seen all kinds of gunk, rust and jellied gas in old tanks. Not one was clean on the bottom.

    I have no doubt that water in fuel is bad but saying that condensation in the tank is the main way it gets contaminated seems silly if you actually think about how much is available to condense. My math isn't perfect but its still more than I've seen from the condensation is bad argument.

    There are other ways water gets in gas, have you ever considered that you might be blaming condensation for water that got in another way?

    My crummy calculation is still looking good.
  14. Smokin

    Smokin Active Member

    Such a doush....
  15. reddog99

    reddog99 Active Member

    Wash, although I would agree that most water in gas tanks gets there by being pumped from a contaminated gas station tank, a major flaw in your calculation method is that it treats the condensation theory as a one-time event. In reality, the condensation would occur continuously over a period of time and build up. We don't see much water-in-gas here in sunny (dry) southern California, but the folks in the eastern part of the country are exposed to conditions favorable to condensation on a continuous basis. :(
    Firesail and Sekred like this.
  16. wash

    wash Active Member

    To improve the calculation of the maximum amount of water available to condense over one tank of gas:

    Assume a range of 330 miles on an 11 gallon fill up and driving 33 trips of 10 miles each, a temperature of 50 C (122 f) and 100% humidity.

    After the first trip you have ~97% gas and 3% air. After the last trip let's call it 0% gas for simplicities sake. The average volume of air in the tank is 50% and you have 33 cycles so the maximum theoretical amount of humid air that can get inside the tank over that length of time is 181.5 gallons which is roughly 33 moles and multiplied by the molecular mass of O2, 1056 grams of humid aid can be in the tank. According to a relative humidity chart I saw, 100% humid air at 50 C can hold just under 100 grams of water per kilogram of air.

    That means 100 grams is the max which won't happen in reality because gas tanks have whole systems designed to control evaporation so the air doesn't change after each trip.

    My rough estimate is that 10 grams would be an incredible amount because the gas cap is designed to hold pressure and even a little vacuum I think. There might never be more than 11 gallons of air entering the system between fill ups (and that could contain roughly 3 grams of water).

    If the gas is absorbing the water over that time and we are burning the gas, some of the water is going out with the gas so at the end of the test, there is probably less than 5 grams of condensed water contaminating the gallon or so left in the tank. My estimate of 0.15% water contamination due to condensation is looking right on.

    What can I say, some times I am a good estimator.

    I think that's a pretty normal use case, it matches my commute beside the humidity. An unusual use case with very short trips, 100% humidity and huge temperature swings could be worse if the gas tank is breathing a lot but your gas would be evaporating away too.

    If anyone can improve on that, go for it. One place to improve could be EPA regulations for the gas cap system or the spec on the test smog stations do. Then you could do some PV=nrt to figure out how much air can get in over a big temperature swing.
  17. Sekred

    Sekred Active Member

    Last edited: Mar 25, 2014
  18. wash

    wash Active Member


    I guess I'm not the only one that smelled something.

    Think for yourself and challenge authority. The worst that can happen is you do some math.
  19. Sekred

    Sekred Active Member

    I'm not totally sold on the maths-yet. I've work on machines with fuel tank capacities up to around 3000 litres and high fuel use, talk in gallons per hour. This gives me a different perspective. I think there are more variables here and I need to do a bit more research before I am convinced condensation is not a issue for light vehicles.
    The other issue and Dave spoke about in his post above, is pump cooling.
  20. wash

    wash Active Member

    The absolute worst case for condensation in tanks is an air compressor. You've got heat cycles, pressure and you're running ~unfiltered air through it at great volume.

    Still in a compressor under heavy use about 500' from San Francisco bay, it goes a few days before a few gallons of water need to be purged from the tank.

    A compressor with light duty, in a much drier location, cycling on and off mostly to replace the air lost to leaky fittings, after several months had about 7 gallons of rusty water at the bottom mostly because it never had a chance to blow any wet air out.

    Those are another two things I've seen that make the gas tank condensation claim seem fishy.
  21. ertzog

    ertzog New Member

    A compressor is passing exponentially more than the single makeup volume of air in a single tank.

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