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Author: legalwordwarrior Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 71227  
Subject: Tire Pressure Warning Light Date: 1/26/2013 10:15 AM
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So I live in the land of changing temperatures (45 degrees last Monday, warmed up to 79 yesterday). Something I have noticed on Stella is that she has a low tire indicator light that comes on when it gets cold, but doesn't go off after I've driven for several miles. On Tuesday, I was planning to add air to the tires, but got sidetracked. Wednesday morning was fairly warm and the indicator light was off and has been off since.

Question: should I increase the pressure a bit anyway?

LWW
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Author: ToddTruby Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69781 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/26/2013 11:02 AM
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I would just make sure they are all the same and at least the PSI indicated on the inside of the door jam. The light is probably indicating a difference in pressure outside the acceptable limits rather than just an overall low pressure.

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Author: pauleckler Big funky green star, 20000 posts Top Favorite Fools Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69782 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/26/2013 2:30 PM
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The pressure you want is the one at driving temperature. So if pressure is low on a very cold morning, I would not worry about it. But if the light is still on after driving say 10 miles (enough to warm up the tires from flexing the rubber), its time to add air.

But maybe if the light is on on a cold morning first should be a walk around the vehicle to be sure none are seriously flat.

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Author: dbruce100 Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69783 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/26/2013 3:59 PM
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Don't forget the spare tire. It also can set the light off.

I usually go a few pounds high on tires. Better mileage and a bit more leeway before I need to fill.

The light generally goes off when you are 5 pounds below recommended. Even with temp changes, the light means you are low on one or more tires.

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Author: legalwordwarrior Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69785 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/26/2013 5:48 PM
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The light generally goes off when you are 5 pounds below recommended. Even with temp changes, the light means you are low on one or more tires.

That's what was so weird though. The light came on while it was cold outside, but as soon as the temps warmed up, the light went off. I didn't add any air to the tires yet.

LWW

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Author: Volucris Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69788 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/26/2013 9:35 PM
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You should definitely check it and see what's up. Maybe your indicator is messed up.

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Author: mmrmnhrm Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69789 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/27/2013 12:02 AM
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That's what was so weird though. The light came on while it was cold outside, but as soon as the temps warmed up, the light went off. I didn't add any air to the tires yet.

What's strange about that? Temp goes up, pressure goes up. You may (or may not?) have had the equation PV=nRT in high school chemistry... pressure * volume = amount of gas * universal gas constant * temperature

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Author: legalwordwarrior Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69790 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/27/2013 9:12 AM
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You should definitely check it and see what's up. Maybe your indicator is messed up.

Checked it and discovered that all 4 tires were at just below 30psi, door label indicated cold psi should be 35. Got DH to add the air, as he doesn't let me play with the air compresser.

LWW

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Author: legalwordwarrior Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69791 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/27/2013 9:16 AM
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What's strange about that? Temp goes up, pressure goes up. You may (or may not?) have had the equation PV=nRT in high school chemistry... pressure * volume = amount of gas * universal gas constant * temperature

I guess it's just that I've never had a vehicle with a pressure indicator light before. It just seemed weird to me that the indicator light would come on while it was cold and never go out, but then suddenly shut off as soon as the temps were in the 70s again. I wouldn't have thought that the outside temp would make that much difference.

And no, I never took chemistry, in high school or college. As a general rule, I avoid anything sciency.

LWW

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Author: pauleckler Big funky green star, 20000 posts Top Favorite Fools Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69795 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/27/2013 4:06 PM
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In PV=nRT, the T is in degrees Kelvin or Centigrade degrees above absolute zero. Room temperature is abt 300 deg K. So a 10% change in tire pressure would require a 30 deg C change in temperature. That's 54 deg F.

So if your tires are at 30 psi at 70 deg F, expect tire pressure to fall to 27 psi when the temp falls to 16 deg F. (That's in theory treating air as an ideal gas.)

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Author: BlueGrits Big gold star, 5000 posts Top Recommended Fools Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69796 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/28/2013 12:14 AM
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I think you're also assuming the volume of the tire doesn't change with the temp. I don't have any idea how much/little that is.

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Author: dbruce100 Big red star, 1000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69797 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/28/2013 8:58 AM
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Wonderful science info thread.

Moral of the story, when you see the tire pressure light......check your tire pressure.

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Author: pauleckler Big funky green star, 20000 posts Top Favorite Fools Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69798 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/28/2013 3:39 PM
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I think you're also assuming the volume of the tire doesn't change with the temp. I don't have any idea how much/little that is.

Yes, the volume can change a bit, and the temperature estimates are not precise. So these are estimates.

Also not that all of this is proportional. If you expect a 3 psi change from 54 deg F, it follows that a 1.5 psi change comes from 27 deg, and 6 psi comes from 108 deg.

So if as previously reported 5 psi low will set off your indicator, it takes a hell of a temp change for that to be temperature alone. Probably it means you were two or three psi low without the temp effect.

Yes, the message is still its time to check your tire pressure. (And sadly for me the light usually comes on after it snows--in messy weather. Better to check tires before the storm.)

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Author: devoish Big red star, 1000 posts Feste Award Nominee! Old School Fool CAPS All Star Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69799 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/28/2013 10:56 PM
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A couple of things:
It is correct that warmed up tire pressure is what matters to driving safely. But for consistency's sake it is cold tire pressure that matters for filling them correctly and then it is presumed that the tires will warm to similar temperatures and correct pressures. All automakers give tire pressures specs for when the tire is cold, not after you have driven it and warmed them up.

Even if it is true that the air volume change is proportional across temperatures it is not true in real life that tire pressure change from different starting temperatures as the tire heats up is proportional.
An under inflated tire will get significantly hotter than a correctly inflated tire because the sidewall flexes more as it rolls to the bottom and flexes as it flattens out. An overloaded tire will also generate more heat.
Both of these conditions will cause a tire to get hotter than it otherwise would and cause the pressure in that tire to change more than a correctly inflated tire will.
But mostly excess heat will cause the tire to come apart, so ultimately if the light is on, check the tires.

And yes I know - we've heard that message somewhere before...

Hope that helps,
Steven

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Author: legalwordwarrior Big funky green star, 20000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: 69801 of 71227
Subject: Re: Tire Pressure Warning Light Date: 1/31/2013 2:18 PM
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So if your tires are at 30 psi at 70 deg F, expect tire pressure to fall to 27 psi when the temp falls to 16 deg F. (That's in theory treating air as an ideal gas.)

Good Lord! 16 deg F.? I am so packing up and moving further South if it gets that cold here again!

But seriously, thank you for the explanation. We filled the tires to the proper cold temp psi and so far no worries. There was frost on the windows this morning and the light never came on.

LWW

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