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What happens when gasoline gets old - chemically and in car engine?

  1. Nov 6, 2014 #1
    Recently I bought an old car (1954 Ford Zephyr, a six-cylinder 2.2 liter English car) whose recent history was essentially unknown to the seller but had probably done little running for some time. He warned me that the engine was not running correctly - the car was blowing black smoke and misfiring under more than a moderately open throttle.

    I checked the ignition system and timing, and dismantled the carb, checked its parts and reassembled with new economy diaphram and gaskets. That car no longer blew black smoke but the misfiring at anything more than 30% throttle, accompanied with spitting back through the carb, was still there unchanged. I was running out of things to check, when it occurred to me that the fuel in the tank might well have been there for some years.

    I drained the tank, put fresh 98 octane lead-free fuel in it and .... the misfiring and carb-spitting was gone. So I am assuming that the fuel was indeed old and had deteriorated.

    It seems widely believed that gasoline deteriorates quickly when stored but definite information seems hard to come by. So far, I have found no information on the following:

    - In what way does gasoline deteriorate when kept in the fuel tank of a car. Chemical changes? Evaporation of volatile components?

    - What happens (or does not happen) in the engine's cylinders when the degraded fuel is used?

    I now have a quantity of gasoline that I can't use in the car. Somebody told me that I could get rid of it simply by adding it to the tank of a modern car that uses fuel injection, rather than a carburetor. Is that correct?

    My thanks for any information or pointers to where I can learn more.
  2. jcsd
  3. Nov 6, 2014 #2


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  4. Nov 7, 2014 #3


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    From 'gasoline FAQ': http://www.faqs.org/faqs/autos/gasoline-faq/part1/


    "Stale" fuel is caused by improper storage, and usually smells sour. The

    gasoline has been allowed to get warm, thus catalysing olefin decomposition

    reactions, and perhaps also losing volatile material in unsealed containers.

    Such fuel will tend to rapidly form gums, and will usually have a significant

    reduction in octane rating. The fuel can be used by blending with twice the

    volume of new gasoline, but the blended fuel should be used immediately,

    otherwise teh old fuel will catalyse rapid decomposition of the new,

    resulting in even larger quantities of stale fuel. Some stale fuels can drop

    several octane numbers, so be generous with the dilution.
  5. Nov 7, 2014 #4


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    The misfiring you report is likely due to the electrolytic capacitor across the ignition points “drying” out over time. Make sure that the distributor breaker points are clean and correctly set. Also you should lubricate the distributor shaft and internal advance mechanisms.

    You can use your old fuel if you mix it 50:50 with new fuel. You will probably need to add an upper cylinder lubricant to your new fuel if the old engine was designed to run on leaded fuel.

    “Stale” fuel makes starting in cold weather difficult because the volatiles have been lost over time. Different countries mix fuels differently, so in some regions of some countries, old fuel can be a problem if not mixed with new.
  6. Nov 8, 2014 #5
    Thank you for the comment. As I mentioned, I had checked the ignition system (I checked spark coil output voltage by using a calibrated spark gap, replaced the capacitor, replaced the rotor arm, checked the timing with a strobe).

    There is no question that the misfiring was due to the fuel and was not an ignition problem. When I added 10 liters (I am guessing with 5 liters of old fuel in the tank) the misfiring was less but still made the car undrivable. When I replaced the fuel completely, the problem disappeared.

    The strange thing was that the car has always started very easily even with the old fuel. It was only under >50% throttle that it misfired.

    I'm still curious to understand:
    - What change happened to the fuel. Chemical? Loss of volatile components? Absorbing water vapor?
    - What happens in the cylinder to prevent proper combustion at more than 50% throttle with the old fuel.
  7. Nov 8, 2014 #6


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    Each day that the tank heats and cools it breaths air in and out. Old cars do not have the charcoal reservoir canister on the vent. After more than a thousand cycles it will have vented the volatile components needed to initiate combustion while also condensing water into the fuel tank. When you start the car it may pump some condensed water into the carburettor float chamber which sets the levels wrong and maybe gives the main jet a water mix. There are many possibilities, an accurate diagnosis is not of value since historical problems are usually multi-dimensional and you seem happy that you have fixed it.
  8. Nov 8, 2014 #7


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    Does gasoline decompose overtime into its combustion products? (Water, CO2)

    It seems like this should happen slowly over time from an energy perspective but I am not sure.
  9. Nov 8, 2014 #8


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    Maybe from an energy perspective, if sufficient activation energy was available. But where will the oxygen come from? The vapour in the top of the tank is saturated with fuel volatiles, so CO2 will displace the oxygen and stop the process, water will sit in the bottom of the tank below the fuel.

    Some diesel fuels will turn to a jelly after a year in a cold tank, but that can be prevented by the addition of for example, a little more methanol, that terminates the chains and prevents them fusing into higher molecular weight lumps.
  10. Nov 10, 2014 #9
    Thank you for all the information. Yes, I am very happy that the problem has been solved.

    I'm still puzzled that the car always started very easily, with no misfiring at all at low engine peeds) but only misfired on more than half
    throttle. Almost the opposite of what I would have expected.

    I have another old car which sometimes goes six months between being used but always starts first time and has no misfiring problems.

    Despite my searching, I have not tracked down what goes wrong with old fuel. Absorption of water? Evaporation of volatile components? Chemical changes? Simple oxidation? Loss of octane rating? There is even a suggestion (but no hard data) that bad old fuel will cause good new fuel to go bad quickly.

    Thank you again for the information provided.
  11. Nov 10, 2014 #10


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