Cold temperature causes tire to deflate?

At this point, it can be only one thing -- a leak. Take it off and inspect the whole thing.

Mine did this a couple weeks ago, also on the rear drivers side. While all tires looked fine in the cold stormy darkness, when I got home I could hear a very faint hiss, and pressure was much too low. Turns out some foreign object punctured the tread, cut some cords, and wrecked the tire, but didn't actually cause any significant leak until it was ejected.

Based on a very slight imbalance somewhere that I hadn't yet tracked down, I think the object was in for a couple months. The cut cords caused a slight bubble, which eventually wore through enough to release the puncturing object and leak more quickly.

 

It's a good thing to know your tires. Until/unless you do, monthly checks are good. A person can tell a lot by looking at the contact areas, but does everyone know what they're seeing when they look?

Same with oil level. It's about checking to ensure all is well, not that you doubt your vehicles quality or ability.

Have you hugged your tires this month?

 

That's good advice on checking the tires and the oil. Sometimes thing go wrong after years of normalcy

I find radials a little hard to judge by appearance so for a quick check I press mid sidewall with my thumb. If I tire is 5 psi low I think I can usually tell the difference. For sure I can feel the difference in sidewall stiffness before I can see a difference.

 

No I honestly haven't hugged my tires but I do have a 100 PSI guage that I use since my tires sit at or near the 50 PSI mark (PSI guagues are only accurate in the midrange and midrange for 60 PSI guauges are too low for my use on the Prius).

It really does pay to check often. I recently had my oil changed at the dealership and forgot to tell them not to drop the tire pressure. They marked on the sheet that they dropped all four tires to 32 PSI but by my guage it was more like 38.

I'm not one to check mine by the calender. I check them by the tempature swings. Around here it isn't uncommon for the high or low temp to vary by 40 degrees in two days time. So I check anytime I hit the really cold temps or the really warm temps.

I watch my MPG enough that I'd notice if one or more tires were low and anytime the MPG drops more than a couple of units I end up checking the front tires at the minimum. I've found over the years that the front tires tend to lose pressure more than the rear though I haven't had that issue with this set of tires so maybe that is another outdated rule of thumb.

 

I've had a lot of experience with tire contact visual cues due to oversized truck tires. They would crown a LOT when overfilled, and wear quickly on the edges when underfilled. Eventually, happy mediums were met.

It's a lot like learning to track animals, I think. You can often tell inflation status by looking at the wet tread trail as you drive through water and then over dry concrete. You can tell by the sidewall droop. You can tell by looking at the dust/rubber boundry at the side of the tread. The long-term inflation is done by tread depth trends. Temperature variables are dialed in as appropriate. New vs broken in is dialed in as appropriate. Experimenting with low/high pressures and subsequent tread observations are performed and filed away for future use.

I observe tires on the cars around me on the road, and make guesses as to the number of PSIs they are low. I honk to get attention and then point to dangerously low tires to inform drivers of their impending doom. Most of them are appriciative.

And to think that some people NEVER put a tire gauge on their stems, and wouldn't know how to operate a gauge if one was placed in their hand. Atrocious!!!

 

My Prius has a tire reset button under the steering wheel. The alarm means that it is some amount below where it was when the button was last reset*. Is this not so on the Gen III?

* - Some people have been known to over-inflate their tires, hit the reset button, then deflate the tires a bit. This makes the alert hyper sensitive. Weather is to flukey here to do that.

 

I am surprised that on this forum nobody has mentioned the physics behind it. It is not magic that tires deflate in the cold, it is because of how temperature and pressure are related. This can be seen in the ideal gas law: Ideal gas law - Wikipedia, the free encyclopedia

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where P is the pressure of the gas, V is the volume of the gas, n is the amount of substance of gas (also known as number of moles), T is the temperature of the gas and R is the ideal, or universal, gas constant, equal to the product of Boltzmann's constant and Avogadro's constant.​

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As you can see, when the Temperature goes down, and you have the same Volume of air in your tires, the Pressure must go down. It is just math.

 

Thanks for the reminder. If anyone wants to do the math, be sure to use absolute pressure, not gauge pressure and absolute temperature, not F or C. For absolute pressure near sea level in USian units add 14.7 psi. For absolute temperature in Rankine (not to be confused with R, the ideal gas constant) add 460 to the temperature in F.

That is where the old rule of thumb of 1 psi change for 10 degrees F comes from.

 

Chemistry's Ideal gasses abide by Gay-Lussac's law, as temperature decreases so does the pressure of the container containing the ideal gas.

 

A faulty Tire Pressure Monitoring System can be a leak at the O-ring in the valve stem sending unit. The dealership replaced mine (on a different Toyota, not a Prius) under warranty and no more leak.

 

It's the same on Gen3. The intruction is a little vague, saying after a reset to "wait a few minutes in the ON mode". Not sure how long that constitutes, I just left it a "decent interval", even more vague, LOL.

 

my left rear tire went wayyy down on saturday night into sunday day. TPS light came on that night. All the tires looked fine so I assume it was the spare because the dealer said that could trigger it. But the next morning I found which one it was.

 

Bad assumption