Run Flat Tires and TPM: a Safety Blessing or Potential Hazard?

[bciDoc]

About 6 weeks ago the TPM light (Tire Pressure Monitor) in my 06 Z4 came on. After checking all tires it turns out the left-rear was almost completely flat. [Personally, I was a bit surprised the TPM didn’t trip sooner.] So I fill it up hoping it’s only a slow leak, reset the light and I’m on my way. At this point I figured if it was a serious leak, the TPM light would come on again and I’d know it was time for new rubber.

Fast forward six weeks: Today I get in the car and as I drive off, I can tell the same tire is now *completely* flat... It's so bad in fact, I not only feel but hear it; yet NO TPM light! What concerns me more is the day before I had taken a spirited drive on a tight-twisty mountain road which lead to a highway run well in excess of the 'recommended' top speeds for a run-flat tire with no air.

The only indication there was a problem was a slight vibration and the traction control light flickering every so often as I powered out of some turns. At the time I thought it was because the roads were damp and it was cold, however in hindsight it was clearly because the tire was seriously low.

So it begs the question, just how safe is it to have run-flat tires and a TPM system which lull you into a false sense of security; allowing you to drive the vehicle beyond the recommended safety limits? I’m a fairly bright guy so I doubt I’m alone in assuming the TPM light should warn you long before things get dangerous, especially in a high performance car that the manufacture knows will be pushed hard.

Seriously... what was BMW thinking? Am I missing something? Am I alone in my opinion here? Has anyone else experienced this? Your thoughts please.

P.S. The moral of the story is, don’t trust your TPM…Use common sense: If you don’t already, carry a tire pressure gauge and check your tires often. I know I will.

[TopDown]

Sounds like a malfunctioning TPM. I've been happy with the wife's e92 RFT/TPM (I don't have it on my M Roadster). First month after she took delivery the TPM signaled low pressure in one tire, which turned out to be 10psi below what I has originally set it at. Brought it into a local tire shop and they found the culprit - a nail. BTW, they fixed the RFT at no charge (so I bought the guys lunch). More recently the TPM went off again, this time for all four tires. I checked the pressure and all four RFTs were down 3-4psi. Just normal pressure loss - I filled all four and they're OK.

I would bring your car to the dealer and have them check your TPM system.

[Rick Hunter]

1) Never trust a TPMS system. Always use a tire pressure gauge (I've got both digital and analog types in my car) and check pressures often.
2) BMW didn't get it "wrong" since it is now a US federal requirement that all new vehicles be fitted with a direct TPMS system. What BMW should have done is integrate sensor monitoring into our OBC display (i.e. allow the ability to display the actual pressures of each tire on our display screen).

The TPMS system employed in our Zeds should engage the flat tire warning light when ~25% of the tire pressure is out of programmed range. Personally I think this range is too great. But then again, the change in pressures also depend on tire temperature while driving. Too bad the TPMS system cannot adjust for this.

It's weird that there were no visual cues on your display that indicated a malfunction in your TPMS system. I wonder if a trip to the shop will help solve the problem?

[Hawkeye]

I have found that my TPM is over sensitive actually. It goes off after only a couple psi of variation (between the tires of course). I have had really good luck with my TPM so far.. knock on wood.

As I have read it it doesn't go off the pressure in your tires. It only goes off if one tire loses significantly more pressure than the same tire on the other side of the vehicle.

[laurenty]

why would you reset the TPMS before fixing your tire???
I don't think that the TPMS actually reads the pressure, but reports a significant loss of pressure from what it was reset at.
So if you reset it while you have a flat tire, you won't get anymore warning.

from the manual:
"In order to assure the reliable reporting of a flat tire, the system must be reset while all tire inflation pressures are correct."

[bciDoc]

Originally Posted by laurenty

why would you reset the TPMS before fixing your tire???
I don't think that the TPMS actually reads the pressure, but reports a significant loss of pressure from what it was reset at.
So if you reset it while you have a flat tire, you won't get anymore warning.

from the manual:
"In order to assure the reliable reporting of a flat tire, the system must be reset while all tire inflation pressures are correct."

The TPM was reset 'after' i filled the tire. The car had sat on the dealer lot for about 6 months before I purchased it; it's not uncommon for tires to gradually loose air overtime from just sitting and the lower they get, the quicker they loose air when under load. It's a bit of a snowball effect. So I wanted to see [hoped] that's all it was before I took it to a shop for inspection and the inevitabe 'you have to replace it'. As I mentioned in my original post, the car was fine for 6 weeks or so, so I thought nothing of it.

For the record, my car has the 'older' style TPM which uses the ABS sensor to tell if one wheel is rotating at a different rpm than others. [if the tires has less air, the diameter is smaller and therefore rotates faster. That is what sets off the TPM; Newer cars have actual monitors inside the wheel,

[Rick Hunter]

Oops, I didn't notice your car was an '06. All '07s and later use the direct TPMS, '06s and before use indirect TPMS. And for the record... the direct TPMS sensors DO detect both temp and pressure. BMW was just too cheap to have it display on our OBC. What a waste of technology...

[imz4n]

X2 what Rick said.

[ZEEFOUR]

BMWs TPM is very basic and often times the tire is down to the run flat before it tells you the tire is low, compared to the more sophisticated systems. I test drove a Corvette Z06 yesterday (which was a hoot to drive!), and the system displayed the individual tire pressure (32 psi in left front) for each tire. Nice feature, but i'm "old school" still check tires with a gauge every 2 weeks

[Master Apex]

Originally Posted by Rick Hunter

Oops, I didn't notice your car was an '06. All '07s and later use the direct TPMS, '06s and before use indirect TPMS. And for the record... the direct TPMS sensors DO detect both temp and pressure. BMW was just too cheap to have it display on our OBC. What a waste of technology...

Mine is 2007, if I buy a set of wheels without TPMS sensor, will the TPMS warning stay lit or can I reset to ignore it?

[Hawkeye]

I am pretty sure without the sensor your light will always be on. You can buy extra TMPS though and have them installed. They aren't TOO expensive.