[BarryJI]

It's unusually cold and pissing down with rain in Los Angeles right now so imagine my distress at getting a low-pressure tire warning as I was on my way to dinner in a slightly sketchy neighborhood this evening. I've had bad luck with picking up nails and I was furious and worried about having to pull in, cancel dinner and spend two hours in the rain waiting for a pickup truck. But then I called up the TPMS screen where it appeared that all four tires (Michelin Summer Sports) had simultaneously lost about 7psi. I concluded that the car was drivable and that the fault was a data output thing rather than a deflated tire. I reset the TPMS which almost immediately cancelled out the fault code. Each tire is about 10 psi lower than the normal pressure value.

Can anyone explain this and reassure me that the car will get me home? Many thanks.

[Poochie]

Originally Posted by BarryJI

Can anyone explain this and reassure me that the car will get me home? Many thanks.

https://www.lesschwab.com/article/tp...heres-why.html

[midlifecrisis]

For most of us in the regions that have four seasons, this has become a late fall ritual. On the first cold morning TPMS warnings light up and everyone adds air to their tires. For one day at least - a greater percentage of the population has properly inflated tires!

[Cabrio330]

Most of the better tire shops in my area have set up drive through tire pressure check lines to handle the seasonal TPMS issues!

[hwntime]

Filling your tires is a part of winter prepping your vehicle in places that have colder climates. Of course it normally doesn't need to be done in SoCal but it has been unusually cool there this past month. I go down to San Diego quarterly for work and a few weeks ago it was in the high 40's at night.

[BarryJI]

Thanks, guys. I am sorry to sound like such a rube but it's the first time this has happened to me in 30 years of driving in SoCal. I am VERY glad that I didn't elect to spend three hours by the side of the road in Koreatown waiting for a tow truck. Now THAT would have made me feel stupid.

[hwntime]

Originally Posted by BarryJI

Thanks, guys. I am sorry to sound like such a rube but it's the first time this has happened to me in 30 years of driving in SoCal. I am VERY glad that I didn't elect to spend three hours by the side of the road in Koreatown waiting for a tow truck. Now THAT would have made me feel stupid.

My old stomping grounds, Koreatown (6th and Harvard). Lived there for 2 years and loved it. It would've been a great place to break down with all the great restaurants lol

[keylime503]

Originally Posted by BarryJI

Thanks, guys. I am sorry to sound like such a rube but it's the first time this has happened to me in 30 years of driving in SoCal. I am VERY glad that I didn't elect to spend three hours by the side of the road in Koreatown waiting for a tow truck. Now THAT would have made me feel stupid.

LA Koreantown sounds like a fantastic place to break down, you can grab some AYCE KBBQ while you wait for the tow truck, what's not to love?

[hwntime]

Originally Posted by keylime503

LA Koreantown sounds like a fantastic place to break down, you can grab some AYCE KBBQ while you wait for the tow truck, what's not to love?

Right! And the beautiful women when the sun goes down. That place is crazy late at night with all the bars and restaurants.

[Sportstick]

Keep in mind, pressure will vary about 1PSI +/- for each 10 degrees Fahrenheit of change. If you inflated your tires when cold to compensate for the pressure reduction, be sure to check them again first thing in the morning (before driving) after the unusual cold snap ends to be sure they are not now overinflated.

[BarryJI]

Yeah, in all fairness there are many worse places in L.A. to break down than Koreatown and the restaurants are indeed great (I was on my way to one).

Sportstick, thank you for the good advice. I always fill my tires to about 5 psi over the value recommended on the door plate so this time I will inflate them only to those values, which should protect me from inadvertent over-inflation.

[Sportstick]

Originally Posted by BarryJI

Yeah, in all fairness there are many worse places in L.A. to break down than Koreatown and the restaurants are indeed great (I was on my way to one).

Sportstick, thank you for the good advice. I always fill my tires to about 5 psi over the value recommended on the door plate so this time I will inflate them only to those values, which should protect me from inadvertent over-inflation.

I would also reconsider that baseline strategy. The pressure on the door jamb label is at GVWR, which is fully loaded with maximum cargo and passenger weight aboard. 5 psi beyond that is quite overinflated for daily use. Suggest review of the manual which I think has recommended daily pressures plus the increment for high speed driving. Generally, your pressures will range in the mid-30s, varying a bit by aspect ratio.

[Cumberlandjames]

Sorry but this seems a little strange to me. My M235 convertible sits in an unheated garage all winter and I've never had a pressure warning. I start it up and warm it up every couple of weeks so if press loss occurred I'd notice it. Temps here in the winter can be -30. Do you have aftermarket mags? I've seen cases where the slipper rims cause minor issues with the bead not sealing well?

Nitrogen fill will remedy as it is impervious to changes due to temp. They use it here to fill snow tires to prevent issues.

[aerobod]

Originally Posted by Cumberlandjames

Nitrogen fill will remedy as it is impervious to changes due to temp. They use it here to fill snow tires to prevent issues.

Nitrogen like dry air (78% nitrogen) and oxygen follow the Ideal Gas Law and expands and contracts at similar rates due to temperature changes. Most of what tyre and automotive places state about nitrogen benefits is a myth, the main benefit is the profit they make on selling it: https://blog.tirerack.com/blog/make-...s-and-myths-v1

The reason nitrogen is used in aircraft tyres is for reduced fire risk due to the oxygen in the compressed air posing a risk if a tyre fails and causes a fuel tank rupture, especially if the undercarriage is already retracted. Formula 1 teams use cylinders of compressed nitrogen as it is readily available and also less risky from a fire perspective.

[Maynard]

There are some real benefits to Nitrogen, but it isn't a panacea, certainly not worth a lot of $$. Main benefit is that it comes clean, w/o water in it - The additional variability from the water makes for larger 'growth' in pressure as they warm up. I also think nitrogen is a larger molecule, so it leaks less thru the rubber. At least for me I notice a significant change that I now don't need to adjust them each month, and when I was at a track day I was seeing less pressure gain than others were reporting (fairly extreme heating on track, so probably due to the lack of water vapor). And it was free with the tires, so all it adds is a 5 min trip to get 'em filled. I'll leave that bit about aircraft/pit lane fires to the pilots on the forum, but I'm skeptical (that would be for pure o2, not compressed air?).

Winter weather (if very low humidity) and a home pump that doesn't load up with water (or a drier inline on your compressor) will get you most of these benefits. Just remember that those small portable pumps really heat up the air, so overfill and then bleed off the extra pressure after it sits and cools a little.

[XutvJet]

Originally Posted by BarryJI

Thanks, guys. I am sorry to sound like such a rube but it's the first time this has happened to me in 30 years of driving in SoCal. I am VERY glad that I didn't elect to spend three hours by the side of the road in Koreatown waiting for a tow truck. Now THAT would have made me feel stupid.

I have to admit, this really made me chuckle. A tow truck because of a TPMS warning? I guess it's a sign of the times where we're so reliant what the car is telling us rather than getting out of the car and looking at the tires and thinking, "They're not flat, thus I can proceed home, check the pressure manually, and add air".

Same goes for driving modes and turning off all the nannies. Lots of older folks (I'm 44) drove in an era of powerful cars with no traction enhancing safeguards. Now many drivers, especially BMW drivers, are utterly fearful of driving with no stability aids on because they are worried that they can't keep control of the car on the street LOL



And yeah, tire pressure temps can vary wildly when temps drop or rise hard. Hell, on a 20 degree sunny day, my driver's side tires will read 3 psi higher than the passenger side only because the driver's side tires are facing the sun when I'm parked at work.

[Unicorn123]

Barry,

No worries; one observation I would make is that nothing replaces looking at the tires regularly. Back in the days when dinosaurs roamed the earth (as my son like to say about my childhood), I learned to do a regular "walk around" on the car and use my calibrated thumb (which still works fine) to press on the sidewall and eventually, a tire pressure gauge (which I keep in the boot on my BMW) if a tire appears to be going flat or has lost pressure. RFT's may be a bit more challenging for the thumb method...

Generally speaking, it is good practice to do a visual check once and a while as GP (Good Practice) on the car and tires. It also helps you correlate sensor readings with visual cues. My two cents worth...

[analemma]

Yes, but..
Do you have TPMS or FTM?
My 230i has FTM, which only looks at the differentials of rotational velocity resulting from variations from initial, calibrated tire radius. I don't think it is as sensitive to temperature as TPMS is, which uses an actual pressure transducer in the tire.
I was under the impression that most of our Canadian cars have FTM rather than TPMS.
Nitrogen is a gas, and is subject to the same physical laws as air (which is 78% nitrogen) See Charles Law. or more PV=nRT. Pressure is directly proportional to absolute temperature if volume is held constant.
(sorry if I'm sounding pedantic. Used to be a pedant!)

Originally Posted by Cumberlandjames

Sorry but this seems a little strange to me. My M235 convertible sits in an unheated garage all winter and I've never had a pressure warning. I start it up and warm it up every couple of weeks so if press loss occurred I'd notice it. Temps here in the winter can be -30. Do you have aftermarket mags? I've seen cases where the slipper rims cause minor issues with the bead not sealing well?

Nitrogen fill will remedy as it is impervious to changes due to temp. They use it here to fill snow tires to prevent issues.

[krhodes1]

Originally Posted by analemma

Yes, but..
Do you have TPMS or FTM?
My 230i has FTM, which only looks at the differentials of rotational velocity resulting from variations from initial, calibrated tire radius. I don't think it is as sensitive to temperature as TPMS is, which uses an actual pressure transducer in the tire.
I was under the impression that most of our Canadian cars have FTM rather than TPMS.
Nitrogen is a gas, and is subject to the same physical laws as air (which is 78% nitrogen) See Charles Law. or more PV=nRT. Pressure is directly proportional to absolute temperature if volume is held constant.
(sorry if I'm sounding pedantic. Used to be a pedant!)

FTM will not recognize all four tires being low at the same time. TPMS will. IIRC, BMW TMPS is also relative, not absolute - when you "reset" it, it then triggers at 7-8PSI below whatever the pressures were when set. My Saab was absolute, and the threshold below my preferred pressure ended up being about 3psi. So annoying frequent warnings.

At least BMW lets you see the actual pressures (on TMPS cars anyway) - my GTI does not as it also uses an FTM style system, and I got a low pressure alert at just about the WORST possible time last year - while evacuating from a hurricane in the middle of the night. And it was just lower temps JUST pushing one tire below the threshold (once I bought a gauge at a truckstop).

Still I think the benefits FAR outweigh the hassles. Nobody checks their tire pressure often enough, and with low profile tires you cannot tell by looking until the pressure is REALLY low, nor will the car drive much differently.