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Thread: Filling Fuel

  1. #1
    Junior Member Dry's Avatar
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    Filling Fuel

    Is it better to have the fuel tank fuller or emptier?
    E.g. if I were to normally fill 1/3 when topping up, should I:
    - wait til it's 2/3 full, and then fill to full, or
    - wait til it's 1/3 full, and then fill to 2/3?
    I don't do a huge amount of miles, maybe one full tank a month if i'm not making longer trips, so a little concerned about fuel sitting in the tank for a while as it goes off over time as I understand it.

    and on a related issue, my fuel filler cap seems kinda loose - if I don't push it really hard it doesn't seal properly and can be pulled off without turning, even after the "clicks". is this normal?
    2011 Jaguar XK 5.0 V8 (NA)

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    Senior Member Daverichardson's Avatar
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    I always fill mine right until the pump clicks off. I do use the highest octane fuel, but can say that even though I only fill up a few times a year I have never felt that the fuel was going off.

    Obviously, if it were to be left standing for a year it may be a different matter!

    My fuel cap certainly cannot be pulled off without turning.
    2006 XK Indigo Blue with Caramel seats and Sable carpets

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    I always fill to 'full', i.e., when the pump clicks off. Never seen any point in doing anything else, even though I don't do a massive mileage in the XK - got 2 other cars in regular use. And I always buy the cheapest 95 RON fuel I can find. 95 RON is what the engine is designed for, and although I understand that it can automatically advance the ign. to gain a little extra performance from higher-octane fuel, the increased performance is slight in relation to the higher cost and it isn't 'kinder' to the engine or 'better' for it. As I said, the engine is designed to work happily on 95 RON.

    And there's nothing 'inferior', 'sub-standard' or harmful about dead cheap supermarket fuel. It's all basically the same stuff, comes from the same wholesalers and has to meet the same standards. As 'Which?' magazine says: '... the differences are in the cocktail of detergent additives added to help keep you car's fuel-injection system clean. These differ from company to company, although even cheap supermarket fuel will contain them. High-octane or 'super' unleaded fuel often has greater levels of cleaning additives, or other ingredients such as friction modifiers which may contribute to to reduced engine wear. Some also claim to improve fuel economy, but unless your car is specifically for octane rating higher than regular UK unleaded (95 RON) - for instance, very high performance or some imported vehicles [which does not include our X150 Jaguars] - the differences will be negligible and you'll get more significant returns simply by adopting a gentler driving style. High octane fuels are also typically 10 to 15p per litre more expensive than regular unleaded.'

    What I do avoid is ever starting my engine and then stopping it again until or unless it has had time to warm up. I have been told that most engine wear occurs at initial startup, when the engine is cold, and as a mechanical engineer I can fully believe that.
    _ _ _

    That 'easily pulled off' fuel filler cap certainly doesn't sound right but I'm afraid I don't know what the cause or the cure might be - sorry!
    Jaguar 2007 4.2 litre XK (X150) convertible
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  4. #4
    Senior Member MarkyM's Avatar
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    I always work on the principal that I don't put any more fuel in than I'm likely to use in a couple of months.

    That said, if I have a fuel/supermarket voucher I'll brim it along with a couple of jerry cans. Therefore it always gets supermarket fuel, except once a year when it gets SUL (also supermarket). Last fill was 92l of SUL at .85ppl (normally £1.15) To that I added a double dose of millers petrol ecomax.

    I turn fuel cap until it clicks and no issues or fault codes, appreciate that it will simply continue to click if turned any more.

    The only reason I'd ever run a car on less than half full is if I were taking it on track or an airfield speed run.
    2014 XK Dynamic R Coupe (no backbox, supercharger pulley upgrade and K&N's)
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    I don't think you'll have any issues either way but fuel is heavy. So why carry more than you need. Also, as you say, it goes off, it takes a while probably 6 months+ for any noticeable difference but again, why let it go off if you can just get more in minutes from every other street corner?
    Each to their own but I tend to put in what I need for the week, that way it's always fresh and I'm not carrying around a pair of bowling balls I don't need.
    2007 S type R, Radiance Red/ Champagne

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    Senior Member Robinus's Avatar
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    It takes a long while indeed for fuel to go off. When filling just don't let your tank go right down to almost empty beforehand if you can help it, otherwise put in the amount you like. I tend to fill it right up as I'm there.

  7. #7
    Senior Member MarkyM's Avatar
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    Quote Originally Posted by MrKis View Post
    I don't think you'll have any issues either way but fuel is heavy. So why carry more than you need. Also, as you say, it goes off, it takes a while probably 6 months+ for any noticeable difference but again, why let it go off if you can just get more in minutes from every other street corner?
    Each to their own but I tend to put in what I need for the week, that way it's always fresh and I'm not carrying around a pair of bowling balls I don't need.
    I do think the weight arguement is a little pointless esp with 500bhp under my right foot.

    35l is half a tank and weighs in at around 25kg IIRC.

    I weigh in at 70kg (1.5kg off Lewis's driving weight) and have done for over 35yrs+

    By your argument anyone that weighs in at 25kg+ more than me (and lets face it there's an awful lot of FB Jag drivers out there) really needs to go on a crash diet?

    Octane loss will take about 3 months btw
    2014 XK Dynamic R Coupe (no backbox, supercharger pulley upgrade and K&N's)
    Peugeot 2008 EAT6 with pan roof and heated seats - new addition

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    A crash diet is a serious commitment to weight saving. Stopping when the tank is half full, less so.
    If you don't think it makes a difference, then try measuring your best mpg at 70 for a few miles with the tank empty and full. You'll probably see a 2/3mpg improvement.
    But as I said, each to their own. There are some people who spec Porsches without a radio to save less than 25kg and there are some who aren't bothered.
    2007 S type R, Radiance Red/ Champagne

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  10. #9
    Senior Member M100 TWO's Avatar
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    I keep it full rather than empty especially over winter to keep condensation at a minimum I know it is a plastic tank but old habits from times when tanks were steel Also I can leave home for a trip and I’m ready to go for three hours before thinking about fuel.

    John
    2010 XKR

  11. #10
    Junior Member Dry's Avatar
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    Quote Originally Posted by MrKis View Post
    A crash diet is a serious commitment to weight saving. Stopping when the tank is half full, less so.
    If you don't think it makes a difference, then try measuring your best mpg at 70 for a few miles with the tank empty and full. You'll probably see a 2/3mpg improvement.
    But as I said, each to their own. There are some people who spec Porsches without a radio to save less than 25kg and there are some who aren't bothered.
    It's a ~2% weight difference, at best, so there's absolutely no way it'll be a ~10% mpg difference. Also my nearest motorway is about 1.5hrs away
    2011 Jaguar XK 5.0 V8 (NA)

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