Z4MC: Cold weather keeps triggering tire pressure monitor?

First winter with the MC. For the past 4 days, every time I start the car in the morning, the TPM lights. I check the tire pressures and they're always the same as last time. I live in Seattle, and it's garaged, so it's not like it gets that cold. High-20s at its coldest.

Also, I'm not really sure how to reset it. I've read the manual on the subject, but it confuses me. Before driving away, I just hold down the button for several seconds, which removes the indicator, until the indicator lights up again. Then I drive away, and the indicator disappears shortly after.

Next morning, this situation repeats.

...and what are the pressures?

Front: 29
Rear: 34

This has been the tire pressure for quite some time, before this TPM thing occurred.

Originally Posted by aestheticstorm

First winter with the MC. For the past 4 days, every time I start the car in the morning, the TPM lights. I check the tire pressures and they're always the same as last time. I live in Seattle, and it's garaged, so it's not like it gets that cold. High-20s at its coldest.
Also, I'm not really sure how to reset it. I've read the manual on the subject, but it confuses me. Before driving away, I just hold down the button for several seconds, which removes the indicator, until the indicator lights up again. Then I drive away, and the indicator disappears shortly after.
Next morning, this situation repeats.

Your 08 like my 07 has the radio frequency tire pressure monitor system that tends to be very temperamental and overly sensitive to minor tire pressure fluctuations. I've had mine come on when driving from sea level (Seattle) to Mount Rainier Paradise 5000 feet.
You'll just need to live with the light on or reset it every time the light starts flashing. If you ignore the flashing light long enough it will stop flashing and just stay on. Of course the next time you start the car the whole ritual begins again.
To reset simply start the engine but don't drive off. Press and hold the console button until the tire symbol lights up on your instrument cluster display. Now drive off and after several minutes the instrument cluster light should go out. But then again maybe it won't

Originally Posted by aestheticstorm

Front: 29
Rear: 34
This has been the tire pressure for quite some time, before this TPM thing occurred.

The TPM system doesn't care what the pressures are; it simply senses changes in pressure since the last time it was reset. As pointed out in the Owner's Manual "The system must be reset each time a tire inflation pressure has been corrected......"
BTW the driver's side door jamb label indicates proper cold inflation pressure to be 30 psi front and rear. I run 32 psi front and 30 psi rear to dial out some of the under steer.

Originally Posted by jmillet

The TPM system doesn't care what the pressures are; it simply senses changes in pressure since the last time it was reset. As pointed out in the Owner's Manual "The system must be reset each time a tire inflation pressure has been corrected......"

Exactly true. And severe cold temps causes pressures to drop more than one expects.

Reset is simple - turn ignition key to the on position and press and hold the TPMS button until the light starts to flash (or becomes steady, dont remember for sure). That's it. Owners manual has the details as well.

Would switching to nitrogen in the tires prevent this? Just curious...

Originally Posted by jmillet

The TPM system doesn't care what the pressures are; it simply senses changes in pressure since the last time it was reset. As pointed out in the Owner's Manual "The system must be reset each time a tire inflation pressure has been corrected......"
BTW the driver's side door jamb label indicates proper cold inflation pressure to be 30 psi front and rear. I run 32 psi front and 30 psi rear to dial out some of the under steer.

Originally Posted by pal

Exactly true...

I think I have to call "shenanigans" on this . I have TPMS in my track tires which I run at a higher psi than my street tires. Since the frequency is the same between the TPMS units, the car doesn't know when I change wheels and I don't have to reset anything to alert the car to this fact. So, if it was the case that the TPMS is only looking for a change compared to previous, then I would have received the TPMS alert when switching from track to stock. Not to mention that, according to your logic, you should be able to completely reset the warning for a completely flat tire (which you can't do...I've tried).

Being as modern TPMS units do actually send the tire pressure (a lot of cars actually display the tire pressure in the instrument panel...like the GT3), I think it's more likely the car is looking for either a range or just a minimum PSI value. Once you're outside of that threshhold, then you get alerted.

Keep in mind that some of the differing opinion may be due to the fact that BMW changed the way TPMS works in 2007 to a unit that actually determines pressure and sends it to the car (as opposed to pre-2006 which figured out if a tire was flat or not by comparing rotational speeds, etc).

I will occasionally go a couple of weeks (one time a month) without driving the car, and invariably the TPMS lights up after a long layoff. Of course the pressures are spot on, so I reset and go on. Maybe it loses its baseline or something after a long-ish layoff. That wouldn't explain it happening daily however.

What's the battery life of these things anyway?

I have had this same issue in both the 2005 and the 2008 that I currently own. From the way it was explained to me from the dealership the system works on detecting a "change" in pressure. That change can be from one tire to the next or outside a range. The TPMS views the 4 tires as 1 system and not 4 individual tires. Any change in the system will set it off such as living in cold climates or a flat. Regardless the system does not know the difference. IMHO i would be better off without it as I think most of us would be.
One last thing, I was told that filling them with nitrogen MIGHT help but don't know anyone who has tried

Originally Posted by epbrown

Would switching to nitrogen in the tires prevent this? Just curious...

Nope.

Using a different gas does not change the fact that in a constant volume, gas changes its pressure with respect to changes in temperature.

Originally Posted by Fast6

Nope.

Using a different gas does not change the fact that in a constant volume, gas changes its pressure with respect to changes in temperature.

Sure, but I would imagine different types of gases have different tolerances to temperature. Any scientists in the room?

Originally Posted by aestheticstorm

Sure, but I would imagine different types of gases have different tolerances to temperature. Any scientists in the room?

Moisture that is present in whatever gas you choose to inflate your tires with has an effect on the way the gas performs and makes the response to temperature changes nonlinear and far less predictable. So when you use your air compressor or your floor pump, you're not putting dry air into your tires. Nitrogen (remember that air is 78% nitrogen!) is dried, so it's not subject to moisture. Dry air would perform just the same as dry nitrogen would perform just the same as dry helium.

Dry gases, constant volume?
P1/T1=P2/T2, end of story

Gases with moisture present?
A bit more complex.

Ideal gas law. PV = nRT or P = rho*R*T, where R is either the universal or specific gas constant. Basically, what's gonna determine the amount of pressure drop with temperature is the nR/V or rho*R term. It's gonna be different for air and nitrogen. For air the specific gas constant is 287 J/(kg*K) and for N2 its 296.8 J/(kg*K). For the density, at 1 atm and 288 K Nitrogen is 1.185 kg/m^3 and air is 1.202 kg/m^3. If we assume that the change is linear in the temperature ranges we're looking at the delta for Pressure in air = 345*delta T, while for nitrogen its 351.7*delta T.

Even though its a bit of a simplification (density is gonna change with respect to Temperature as well), we can still safely say that nitrogen is gonna drop more pressure with respect to change in temperature, than air.

Now, the argument to use nitrogen is that molecules take up more space than the little bits of air, that will be lost through the tire over time. Now, how much of the little bits of air are lost is up for debate, since the really small particles in air occupy much less than 1% of air by volume.

Does that help?

http://www.bimmerfest.com/forums/sho...d.php?t=159130

funny thing, that happened to me yesterday when it dropped down to our low of..... *gasp* 39 deg F

New tires put on early fall...mounted and filled with nitrogen at NTB. Here in North Texas we've been having constant back and forth temperature fluctuations of 50 degrees within a day, day and a half (20-70-20-70). All the people here are sick. But my TPMS has made narry a peep.

Originally Posted by jragan

New tires put on early fall...mounted and filled with nitrogen at NTB. Here in North Texas we've been having constant back and forth temperature fluctuations of 50 degrees within a day, day and a half (20-70-20-70). All the people here are sick. But my TPMS has made narry a peep.

Didn't I just tell you that nitrogen pressure will fluctuate more with temperature than air...

Just because your TPMS doesn't go off anything doesn't mean that it doesn't fluctuate, it just means that it's not dropping enough to set off the TPMS.

Originally Posted by Dammmittt

Didn't I just tell you that nitrogen pressure will fluctuate more with temperature than air...

Just because your TPMS doesn't go off anything doesn't mean that it doesn't fluctuate, it just means that it's not dropping enough to set off the TPMS.

Thank you for proving my point. I was purposefully over-simplifying in my last post. I never stated that my pressures weren't fluctuating, just that the TPMS alert wasn't sounding. Everyone here keeps saying that the TPMS works off of a simple difference between checks and that any difference that causes an alert can be re-set. Which is false. My point is that your pressures can fluctuate greatly as long as the minimum pressure doesn't go below a pre-determined threshhold (29psi?). Once past that point you can't re-set the alarm (it will immediately re-alert).

Originally Posted by jragan

Thank you for proving my point. I was purposefully over-simplifying in my last post. I never stated that my pressures weren't fluctuating, just that the TPMS alert wasn't sounding. Everyone here keeps saying that the TPMS works off of a simple difference between checks and that any difference that causes an alert can be re-set. Which is false. My point is that your pressures can fluctuate greatly as long as the minimum pressure doesn't go below a pre-determined threshhold (29psi?). Once past that point you can't re-set the alarm (it will immediately re-alert).

Ahah! I didn't read the whole thread, still haven't.