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Thread: ''Check tire pressure'' light is on in my car

  1. #1 ''Check tire pressure'' light is on in my car 
    ...matter and pixie dust wegs's Avatar
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    One of my dashboard lights popped on today, which alerted me to check my car manual, as to what it indicates. Apparently, my tire pressure is low. The odd thing is, to 'look' at the tires, they don't appear to be low. Here's my question; the manual states to check the tire pressure when they are cool. Why is it necessary to wait for the tires to cool down to check the tire pressure? TIA for your advice.


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  3. #2  
    Forum Radioactive Isotope sculptor's Avatar
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    tires build up pressure when they are hot
    so filling them then to the recommended pressure will actually under fill them slightly.

    get yourself an accurate tire pressure gauge and take good care of it and it will keep you safe
    use it often
    It only takes a couple minutes to check with your gauge.


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  4. #3  
    ▼▼ dn ʎɐʍ sıɥʇ ▼▼ RedPanda's Avatar
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    'Hot' tyres have a higher PSI than 'cold' tyres.
    Any change in temperature will effect the air pressure in your tyres.
    The warmer, the more pressure; the colder, the less pressure.
    But the manual needs a base-line to give a suggested PSI value, but it is still inaccurate as even the outside temperature will affect your tyre pressure.

    The same effect is seen in squash.
    The rubber ball doesn't bounce properly until it has been warmed up (usually by hitting it against the walls of the court).
    But the air pressure in the ball is expected to warm up while being hit and so it is not designed to bounce properly until it is warm.
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  5. #4  
    ...matter and pixie dust wegs's Avatar
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    Quote Originally Posted by sculptor View Post
    tires build up pressure when they are hot
    so filling them then to the recommended pressure will actually under fill them slightly.

    get yourself an accurate tire pressure gauge and take good care of it and it will keep you safe
    use it often
    It only takes a couple minutes to check with your gauge.
    would you judge me if i said i have no idea what a gauge is or how to use it? :=O thank you for your reply, ok i understand as to the theory behind checking them when they're cool.

    Quote Originally Posted by RedPanda View Post
    'Hot' tyres have a higher PSI than 'cold' tyres.
    Any change in temperature will effect the air pressure in your tyres.
    The warmer, the more pressure; the colder, the less pressure.
    But the manual needs a base-line to give a suggested PSI value, but it is still inaccurate as even the outside temperature will affect your tyre pressure.
    really?? the pressure outside the tires, will affect the pressure?

    The same effect is seen in squash.
    The rubber ball doesn't bounce properly until it has been warmed up (usually by hitting it against the walls of the court).
    But the air pressure in the ball is expected to warm up while being hit and so it is not designed to bounce properly until it is warm.
    that's wild! wow!
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  6. #5  
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    Quote Originally Posted by wegs View Post
    ...would you judge me if i said i have no idea what a gauge is or how to use it? :=O thank you for your reply, ok i understand as to the theory behind checking them when they're cool. ... .
    dunno 'bout UK, but here, if you buy a gauge from an auto parts store, and ask for a demonstration, you would get a lesson tossed in free after the purchase.

    or
    have someone else test the tires for you.
    but, really, this is something you should know.

    judge? ignorance? no not me------unless it seems willful ignorance
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  7. #6  
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    They want cool tires because according to Gay-Lussac's law the warmer tires will hold more pressure and give a false reading.

    --
    A visible check is only really effective when you half or less than the required pressure. It's good to check for flats, but not for checking whether your tires are low.

    If you check their pressure they are all ok, you have a bad sensor. Unfortunately there's no easy way for an more owners to know for which sensor which will mean a trip to a tire center, or a well equipped general maintenance garage.
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  8. #7  
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    Ladies may not quite meet the strength requirement, but after some practice, one can actually judge a tire being significantly low in pressure by pushing a thumb against the sidewall. It comes with a bit of practice, but if one carries a tire gauge in the glove box, as they should, and uses it to reduce the pressure in one tire just perhaps 10 psi lower than the other three, it is fairly easy to judge the difference in sidewall "feel" if a thumb is firmly pressed against it. When embarking on a drive after the vehicle has not been in use for some time, it is good practice to simply glance at all four tires, noting the "bulge" where they sit on the pavement. If one seems to bulge more than the others, the "thumb" test can readily warn you of impending trouble. If it feels adequately "stiff" to the thumb, it probably is OK. If not, get the gauge out! I have many times "caught" a low tire in this way and likely avoided plenty of inconvenience, especially since it was usually my wife's car involved, just before she left for work!

    Radial-ply tires tend to always have a visible bulge at their bottom regardless of inflation level, so just "guessing" by the bulge, is inviting trouble. jocular
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  9. #8  
    ▼▼ dn ʎɐʍ sıɥʇ ▼▼ RedPanda's Avatar
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    Quote Originally Posted by wegs View Post
    the pressure outside the tires, will affect the pressure?
    It is primarily the temperature that affects the air pressure inside the tyres - heat/cold from the weather and heat from the friction between the tyre and the road.
    (Changes in air pressure outside the tyres should have an effect on tyre pressure - but I expect the change is small enough to ignore - unless you are planning on driving up a mountain.* )

    *An educated guess - not got time to google confirmation.
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  10. #9  
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    When I used to drive an 18-wheeler it was expected I would check the pressure in all my tires every day as part of my daily inspection. But using a gauge on 18 tires takes longer than is necessary, professional drivers carry a thump stick and just give each tire a whack. Tires with low pressure give a distinctly different sound than properly inflated ones, at least the high pressure tires used on 18 wheelers do.
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  11. #10  
    ...matter and pixie dust wegs's Avatar
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    Quote Originally Posted by sculptor View Post
    Quote Originally Posted by wegs View Post
    ...would you judge me if i said i have no idea what a gauge is or how to use it? :=O thank you for your reply, ok i understand as to the theory behind checking them when they're cool. ... .
    dunno 'bout UK, but here, if you buy a gauge from an auto parts store, and ask for a demonstration, you would get a lesson tossed in free after the purchase.
    I live in the US, I may need to do this. But, then again, I'm going to take the car to the tire shop, where I buy my tires and let them worry about it. I mean, what's next...I'll be changing the oil? Fixing the transmission myself? That's why there are mechanics and tire shops. I'm fine not knowing what a tire gauge is.




    judge? ignorance? no not me------unless it seems willful ignorance
    It might be willful in this instance. Thank you your kind help.

    Quote Originally Posted by Lynx_Fox View Post
    They want cool tires because according to Gay-Lussac's law the warmer tires will hold more pressure and give a false reading.

    --
    A visible check is only really effective when you half or less than the required pressure. It's good to check for flats, but not for checking whether your tires are low.

    If you check their pressure they are all ok, you have a bad sensor. Unfortunately there's no easy way for an more owners to know for which sensor which will mean a trip to a tire center, or a well equipped general maintenance garage.
    I appreciate this Lynx, I've learned something new today. I honestly never knew about tire pressure, cold vs hot tires...etc. When you take your car to the shop, or you have the tires checked when you go to full service gas stations (yes they still exist) you just don't think about it.

    Quote Originally Posted by jocular View Post
    Ladies may not quite meet the strength requirement, but after some practice, one can actually judge a tire being significantly low in pressure by pushing a thumb against the sidewall. It comes with a bit of practice, but if one carries a tire gauge in the glove box, as they should, and uses it to reduce the pressure in one tire just perhaps 10 psi lower than the other three, it is fairly easy to judge the difference in sidewall "feel" if a thumb is firmly pressed against it. When embarking on a drive after the vehicle has not been in use for some time, it is good practice to simply glance at all four tires, noting the "bulge" where they sit on the pavement. If one seems to bulge more than the others, the "thumb" test can readily warn you of impending trouble. If it feels adequately "stiff" to the thumb, it probably is OK. If not, get the gauge out! I have many times "caught" a low tire in this way and likely avoided plenty of inconvenience, especially since it was usually my wife's car involved, just before she left for work!

    Radial-ply tires tend to always have a visible bulge at their bottom regardless of inflation level, so just "guessing" by the bulge, is inviting trouble. jocular
    This is fascinating, I never knew this...I will have to 'practice' ...If a light is coming on in my car, I've become woefully neglectful of taking care of my tires as I should. Appreciate your advice.

    Quote Originally Posted by RedPanda View Post
    Quote Originally Posted by wegs View Post
    the pressure outside the tires, will affect the pressure?
    It is primarily the temperature that affects the air pressure inside the tyres - heat/cold from the weather and heat from the friction between the tyre and the road.
    (Changes in air pressure outside the tyres should have an effect on tyre pressure - but I expect the change is small enough to ignore - unless you are planning on driving up a mountain.* )
    hahaha No mountains where I live. Again...how come everyone here knows so much about tires? lol

    *An educated guess - not got time to google confirmation.
    I have a feeling, it's a good guess.

    Quote Originally Posted by danhanegan View Post
    When I used to drive an 18-wheeler it was expected I would check the pressure in all my tires every day as part of my daily inspection.
    I'd have to quit that job, then
    But using a gauge on 18 tires takes longer than is necessary, professional drivers carry a thump stick and just give each tire a whack. Tires with low pressure give a distinctly different sound than properly inflated ones, at least the high pressure tires used on 18 wheelers do.
    I cannot believe all I've learned about tires today. Thank you for chiming in, as well. You don't drive a truck for a living anymore? That seems like it would be challenging to do day in, day out.
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  12. #11  
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    [QUOTE=danhanegan;461161]When I used to drive an 18-wheeler it was expected I would check the pressure in all my tires every day as part of my daily inspection. But using a gauge on 18 tires takes longer than is necessary, professional drivers carry a thump stick and just give each tire a whack. Tires with low pressure give a distinctly different sound than properly inflated ones, at least the high pressure tires used on 18 wheelers do.[/QUOTE

    The tires involved here are gigantic compared to folks' car and SUV tires. As such, they carry MUCH higher pressure, usually around 100 psi, have enormously stronger sidewalls, forget about any "thumb" check there, and when Dual-Mounted tires are involved, the inside-mounted of the dual-tire combinations sometimes DEFY use of a tire gauge; special truck-tire gauges are used having offset and backwards-facing openings. But that's specialized stuff. Most folks here concerned with avoiding tire trouble drive cars, pickups, or SUVs. The safety "check" rules apply equally to all of those. Always remember that the vast majority of tire failures occurring at speed on highways are caused by being run while low on pressure. Such operation results in high temperatures generated in the tires' internal structure, this eventually causing tire failure. jocular
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  13. #12  
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    Hot tires don't show accurate pressure reading.
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  14. #13  
    ▼▼ dn ʎɐʍ sıɥʇ ▼▼ RedPanda's Avatar
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    Quote Originally Posted by babe View Post
    Hot tires don't show accurate pressure reading.
    I don't think it is a case of 'accuracy' per se, more a case of them trying to ensure you are comparing like with like.

    Think of it like the heating instructions for food.
    They say "pre-heated oven" mainly* because ovens take different amounts of time to reach the required cooking temperature.
    To ensure the cooking time is accurate (which can be very important for meat), they measure the time starting from a known point (i.e. the oven is at 180oC).

    Also, due to the litigious nature of the public, it might be considered dangerous to tell people to test their tyres when they are hot.
    A little bit like saying "To test your parachute, jump out of a plane and pull the cord".
    They don't want to be telling people to go for a half-hour drive before testing that their tyres are safe.

    *There are also some additional reasons (to do with Maillard reactions, etc.).
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  15. #14  
    Forum Radioactive Isotope sculptor's Avatar
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    Maillard reactions
    and tire pressure?
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  16. #15  
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    Quote Originally Posted by sculptor View Post
    Maillard reactions
    and tire pressure?
    Thanks and *chuckle*
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  17. #16  
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    Quote Originally Posted by sculptor View Post
    Maillard reactions
    and tire pressure?
    Begging your pardon, but WTH are these? joc
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  18. #17  
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    One of my dashboard lights popped on today, which alerted me to check my car manual, as to what it indicates. Apparently, my tire pressure is low. The odd thing is, to 'look' at the tires, they don't appear to be low. Here's my question; the manual states to check the tire pressure when they are cool. Why is it necessary to wait for the tires to cool down to check the tire pressure? TIA for your advice.
    In fact, tyres, (or tires, if you prefer), can look correctly inflated when they're not.

    A gauge is used to check the pressures. Just unscrew the dust cap and push the gauge firmly onto the valve. Doing this opens the valve and the gauge will show the pressure inside the tyre. Do this before you drive the car and when the air is cool.

    A lot of petrol (gas) stations have automatic tyre inflators now - you set the pressure you want, then push the hose onto each valve in turn. The inflator will fill each tyre to the correct pressure, then beep - then you move onto the next one. Do the spare tyre as well, and remember to put all the dust caps back on before you drive away.

    I think that tyre pressure warning lights work by 'looking at' the temperature of the tyre, (please correct me). An under pressure tyre will get warmer because the side wall will flex much more with every revolution, and the friction of the flexing will heat the rubber up.


    Measuring a tyre when it's hot will result in you filling it to a lower pressure than it should be, and then the tyre will not provide the correct contact patch and therefore give incorrect grip with the road. The internal tyre pressure will increase with heat, but the manufacturers specify the pressure as measured cold.

    OB
    Last edited by One beer; October 23rd, 2013 at 01:05 PM.
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    Quote Originally Posted by One beer View Post
    I think that tyre pressure warning lights work by 'looking at' the temperature of the tyre, (please correct me). An under pressure tyre will get warmer because the side wall will flex much more with every revolution, and the friction of the flexing will heat the rubber up.
    That's not one of the ways mentioned here.
    Tire Pressure Warning Light

    The computer controlled tire warning system—also known as the tire pressure monitoring system (TPMS)—uses several different methods to track the tire pressure. Some use the anti-lock brake sensors (a deflating tire will turn faster than a properly inflated tire and therefore trigger the light). Some systems use sensors in the wheel wells that carefully track the diameter of the tire (a low tire will have a smaller diameter than a properly inflated tire). Other systems use pressure sensors inside the tire itself. When the tire pressure falls below a certain point, the sensor will send a signal to a transducer, which alerts the tire warning system and illuminates the light.
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  20. #19  
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    Quote Originally Posted by RedPanda View Post
    The same effect is seen in squash.
    The rubber ball doesn't bounce properly until it has been warmed up (usually by hitting it against the walls of the court).
    But the air pressure in the ball is expected to warm up while being hit and so it is not designed to bounce properly until it is warm.
    That is a real drag when you suck at squash.
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  21. #20  
    ...matter and pixie dust wegs's Avatar
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    btw, the problem was a nail in my tire. where are nails coming from? lol
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  22. #21  
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    Quote Originally Posted by wegs View Post
    btw, the problem was a nail in my tire. where are nails coming from? lol
    Did that too....where it came from????? slow leak...what a PITA!!!
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  23. #22  
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    I recommend this tire guage. I've had one like this for over 20 years now. It is more expensive but will last you for a very long time giving you accurate readings everytime. It has a small pushpin that allows you to remove excess air if you overfill your tires.

    http://www.google.com/aclk?sa=l&ai=C...rce%3DMS-5510B

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  24. #23  
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    Yeah, I might be wrong about the temperature method of pressure sensing - hence my question mark.

    However, by coincidence I was just reading my car's manual, and it states that tyre pressure increases by 0.1 bar for every 10 degree increase in temperature.

    0.1 bar is about 1.5 psi.

    Punctures often occur in the rear tyres: The front tyre harmlessly runs over the prone nail, but kicks it up from the road surface and then it sticks in the rear tyre.

    OB
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  25. #24  
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    Quote Originally Posted by One beer View Post
    Yeah, I might be wrong about the temperature method of pressure sensing - hence my question mark.

    However, by coincidence I was just reading my car's manual, and it states that tyre pressure increases by 0.1 bar for every 10 degree increase in temperature.

    0.1 bar is about 1.5 psi.

    Punctures often occur in the rear tyres: The front tyre harmlessly runs over the prone nail, but kicks it up from the road surface and then it sticks in the rear tyre.

    OB
    Damn front tires!
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