jawlee

Never had runflat tires in my life and I haven't had my c43 for long.
This morning my car's TPMS sensors went crazy. I'm assuming due to temperature changes but it was only 10 degrees here in Toronto.
The message had 3 tires showing in yellow. Initially the front two, and one rear tire. 36, 36, 30psi respectively.
Then as i drove a bit, the other rear tire went down to 32 psi but it stayed in white font.
I found a safe spot and went to check the tires. No obvious bumps, cracks, punctures or hissing sound. Didn't look flat.

Just have a few questions:
1) I understand run flats allow you to drive a certain distance after they've "blown", but how do I know when they've actually "blown" and I need to get it replaced ASAP? I'm assuming the 30psi isn't exactly an indicator, but just something i thought of and am not sure how to tell if something were to happen. My assumption is that in that case, it would lose air until it hits 0psi still, and runflat just really means you can drive on that for about 75km or so.
2) Why is it that only my front two tires and one rear tire is shown in yellow?

vincheung

if they are blown, you'll have significantly lower tire pressure if any. In this particular case, it sounds as if the cold weather caused the drop in PSI. I wouldnt worry about why only one two or three of the tires indicated, just top them off while cold and be done with it.

jonathan358

Most likely due to cold weather, your dealer probably set the values at 42/42/38/36 or something like that (values at full capacity -- different for every car).

As the temperature dips, your tire pressure decreases hence the yellows.

When you blow a tire, RFT or regular, the values drop to 0 and you get a message in red as well as the numbers. Yellow is just a warning that your tires are a lot lower than what you (or your dealer) set them at.

If you don't carry passengers often, I would lower the pressure anyway by following that of the gas cap placard.

jonathan358

^ You probably lost air pressure in the meantime as air temperature changes in Toronto will only affect tire pressure by ~5psi max. Not including driven time.

jawlee

Thanks for the help guys!

One more dumb question, our tires are not nitrogen-filled right?
I don't see the valves green or anywhere in the owner's manual saying otherwise.

sieglo

Most likely temperature change. Ideal gas law -- basic science intuitively understood by almost everyone outside the NFL.

jonathan358

They are 78% nitrogen!

Mr. J

Moisture content increases P variability significantly and it's not easily quantified with linearity like the simplified gas laws imply -- intermediate science understood by very few outside of the physical science and engineering communities

IMO N2 for tires on a daily driver is kinda silly. The main reason for using it is to displace all the water vapor so that increases in P are predictable and proportional to increases in T. There is also a tiny decrease in permeability but IMO this is inconsequential.

N2 makes sense for serious track machines where tire pressures are critical and need to be maintained within narrow spec ranges. N2 is also useful for some aircraft and heavy equipment applications.

The takeaways:
1. Check P cold and set your TPMS to these setpoints.
2. Don't worry about minor P fluctuations that affect ALL your tires. For example, if you drive up to the mountains to go skiing and you notice that all your tires dropped 3PSI this is just environmental and not a concern. However, if you're on a road trip and you get a tpms warning that a single tire is low vs the opposite side, then you probably have a leak.
3. Don't trust cheap P gauges. Make sure the ones you're using are accurate. It's not unusual for one gauge to read high and another to read low. This is OK as long as they aren't fluctuating. As soon a a gauge becomes unreliable toss it.
4. Don't try to fill the air by the cars TPMS, there is lag time between the filling and the reporting on the dash display. Always fill and measure the air with a manual gauge. You should always keep a reliable tire pressure gauge in your car.