Does petrol go off over time?

Discussion in 'Motorhome Chat' started by keith, Feb 11, 2016.

  1. keith

    keith Funster

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    Something that crops up from time to time, as it did in a recent thread, so make your own mind up from this.

    Does petrol go off over time?

    All sorts of problems are blamed on petrol that's "gone off" either in a bike's tank or in a separate petrol can. Is this really a problem, and, if so, how should you combat it? We've heard the same about brake fluid, so is that the case too?

    Once again, it's over to the cleverest bloke we know, John Rowland, Fuchs Silkolene's top automotive research chemist to cut through the internet hype and give us the facts.

    "Petrol in a sealed metal can does not go off, provided it is purchased in the UK or any other more or less civilised place. It will be OK for years. About the only additive in fuel is the detergent which doesn't evaporate. In these lead-free days, octane number depends on the molecular make-up of the fuel, and all you need to do is stop evaporation. Even the smallest molecules in there (such as hexane) can't get through metal. Plastic, as used for fuel containers, is itself made up of long-chain hydrocarbons (usually high-density polyethylene) which have similarities with the short-chain petrol hydrocarbons, so small petrol molecules worm their way into the relatively large spaces between the plastic molecules and get out the other side. Losses can be several percent over long periods (months) but thick plastic helps reduce this. You can't beat metal with its compact atomic structure for storing petrol."

    "Petrol is OK in a can with an airspace; a tiny amount of the more volatile fractions of the fuel will evaporate to fill the airspace, and that's the end of it. It becomes a 'closed system in equilibrium'. Molecules go back into the liquid phase at the same rate as they leave it and enter the gaseous phase. Even a three-litre airspace in a five-litre can will only contain a very small amount of fuel vapour, equivalent to one or two CCs of fuel depending on the temperature."

    "As for a vented fuel tank like most bikes have fitted, there will be some minor vapour loss of the more volatile fractions, such as hexane, but nowhere near as bad as losses from a container exposing a large sufface area of fuel to the air - such as a frying pan."

    "Volume changes in the tank airspace due to daily temperature changes cause the tank to 'breathe', but one breath every 24 hours isn't so bad. Even a few months of this with a half full tank isn't a big deal. I get the impression from some bike owners that they think the fuel tank gasps away like an elephant running a marathon. In reality, it's more like a hibernating dormouse. Imagine a 16-litre tank with an eight-litre airspace, starting at 10C it cools to 0C. Using Charles' law, we can calculate the tank would draw in 300cc of air, and blows it out again when it warms up to 10C. But, petrol vapour is heavy, and the air drawn in will tend to sit on top of it, so, when air is lost when the tank warms up, it will be mainly the top layer with only a small amount of fuel in it."

    "As for petrol evaporating from the carbs, some fuel, particularly from non-mainstream suppliers, can leave a tacky deposit when it dries out, but this was more of a 1990s problem. You don't hear of it happening today where civilisation (as we know it) exists. Even so, it's a good idea when putting a bike in storage to add a drop (5cc/litre) of good two-stroke oil to the fuel, run the engine until it's good and hot, shut down and cover with a thick blanket (cuts temperature fluctuation). Then, if the fuel dries out, a protective oily film will be there which is easily washed off when the bike is run again."

    "Brake fluid (avoid the cheap stuff) will keep for years in a sealed, full, plastic bottle. Even a part-empty bottle is OK if the top is on tight. People worry too much about this water-absorption business. OK, it happens, but it's a slow process. Anyway, the boiling point of damp brake fluid is around 190C, so, unless you're out to bust the lap record at the IoM, it's not going to let you down."

    In summary, fuel and brake fluid are unlikely to go 'off' unless stored incorrectly for years rather than months.


    So there you go. If anyone has any old petrol that they think is useless - give it to me please [​IMG]
     
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  2. Khizzie

    Khizzie Funster

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    Certainly makes a so called motorcycle mechanic look a wally ,as he informed me that all petrol is designed to degrade over a period of six weeks after that if you have not used it you must take it to the recycling centre to dispose of...glad I ignored that verbal diarrhoea... Also wont be using that guy to do any work on my bikes ..Roy
     
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  3. Polar bear

    Polar bear Funster

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    I mix two stroke for strimer and chain saw if mixture is left in a sealed container for a few months it will start the above whereas freshly mixed works perfectly OK.
     
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  4. Judge Mental

    Judge Mental Funster Deceased RIP

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    I believe so....going by our lawnmower
     
  5. f6c

    f6c Funster

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    Yes it does go off:whistle:
     
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  6. DuxDeluxe

    DuxDeluxe Funster Life Member

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    He is right to an extent, but petrol (gasoline) can deteriorate over time due to oxidation, which is why it is tested for in the EN specifications before being sold either in bulk or into retail use on a batch basis.
     
  7. Lynne Steele

    Lynne Steele Funster

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    We always drained off the petrol left in our boat for the winter period, or it went like sludge
     
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  8. keith

    keith Funster

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    Didn't he suggest he would take it for you. :rofl::rofl:
    I'm still waiting for all your duff petrol:rofl:
     
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  9. scotjimland

    scotjimland Funster Life Member

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    Rightly or wrongly .. I've never ever drained my Honda mower.. .. after the winter it starts first pull ... same with the Honda genny..

    since forums began I have had to 'unlearn' so many things ... and discovered I've been doing stuff wrong all these years

    ignorance was once bliss...:confused:
     
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  10. Gromett

    Gromett Funster

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    I will test this theory out over the weekend. I have a tank of petrol that I haven't opened in almost 2 years. I will give the genny a go. It has not been started for almost as long. Might be a bit hard to start after all that time though >.<
     
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  11. Charlie

    Charlie Funster

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  12. DuxDeluxe

    DuxDeluxe Funster Life Member

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    Yep - it can oxidise. Even worse, if there is moisture present then you can get the dreaded anaerobic bacteria and sludge.

    Edit - I don't want to cast doubt on the guy's knowledge but I always thought that Fuchs were a lubricants company.
     
  13. Lenny HB

    Lenny HB Funster

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    Yes definitely goes off, but you can add a fuel stabiliser then it can be kept for up to 2 years. Unfortunately one of the best stabilisers Gold Eagle's Sta-Bil is no longer available in the UK, used to be one of my best selling products until my mate stopped importing it (basted).
     
  14. SuperMike

    SuperMike Read Only Funster

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    The trouble with old petrol and carberators left in the likes of lawn mowers and genies etc is the tiny jets. The little bit of fuel left in them can gum a little and block the jet and then you are stuffed.
     
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  15. Lenny HB

    Lenny HB Funster

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    A good fuel stabiliser stops that, after you have added it to the fuel you run the engine for bit.
     
  16. Malcolm Bolt

    Malcolm Bolt Funster Life Member

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    I have problems with the scooter every time I try to start it. Now two years old it had only done two hundred miles
    Next time I get it going I will drain it down and carry new fuel in a sealed can. That could be a while. The scooter is at home cos no room in the van.
     
  17. Steve

    Steve Funster Life Member

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    well, i am a yes and a no.
    i have had mowers that have had old fuel in and ran with no problem, but i have had mowers with old fuel in that had to have carb striped and cleaned. The same with some of my motorbikes. One of those things, you pays ya money and takes the chance. But when at home you can get it sorted. When not and things go wrong bigger problem. Genny in back of van been there for a year pull it out and it starts first time..During the day! pull it out at night when you have a problem and Mr Sod's law steps in.
     
  18. laneside

    laneside Funster

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    Well when our neighbours were over he gave me a gennie that was in the barn when he bought the place eleven years ago. I wire brushed the spark plug and gave it a pull and away it went on the petrol in the tank. I realise it isn't a Kipor
     
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  19. mitzimad

    mitzimad Funster

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    i pulled a car out of a garage that had been in there 12 years and it started with just a fresh battery and a bump start
     
  20. GWAYGWAY

    GWAYGWAY Funster

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    I took my CG125 Topokita out in Gerty for a trip to Cornwall, I got it out to ride down into Mevagisey , it coughed and farted all the way down and hardly made it back up the hill, the petrol in the tank had lost some of its vitals, and it had gummed up the main and needle jets.
    It had been standing for 8 weeks and it was down to the petrol having ethanol and drawing water from the air.
    (PS Topokita is Walter Mitty's machine of every and whatever type and refers to the noise they all make)
     
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