A few nights ago I walked out to my car and started it up. Immediately, I received the warning light and message from the BMW TPM (Tire Pressure Monitor system): Left Front Tire Pressure Low.
My first two thoughts were:
- Ugh, I have somewhere to be in 20 minutes and
- Maybe I should have really considered that tire / wheel insurance since new Michelin Pilot Sport PS2 in the 245/35ZR19/RF 93Y tires run $385 a pop even at Costco!
I clicked through the message on the BMW iDrive system in my e90 M3 and read through the run flat tire (RFT) message which suggested that I did not drive over 50mph while my tire were flat. Just to check, I jumped out of the car real quickly to kick the tires (literally) to see if I could notice lower pressure. I couldn’t tell any kind of difference and the right front tire sure did not look low. But, I did not know if that was due to the run flat tires or not.
I drove carefully over to the local gas station to see if I could put some air in the tire. It was not dark, but the best I could tell is that I was able to raise the PSI to about 40 on the tires that state a maximum of 51 PSI. It did not seem to be losing air for the first couple of minutes I was there. I reset the BMW TPM setting and was on my way, hoping that I could make it to where I was going and that it was either a small leak or the TPM system was not accurate.
I made it to the location without issue where the car sat for 4 hours before I went to drive home. When I walked out to the car, I was not sure what to expect – but the TPM sensor still was not back on. Good news, I drove home and figured I would check things out in the morning.
When I woke up in the morning, I decided to go and check the tire pressures all the way around. 42 PSI on the front left tire, 29.5 PSI in the back left, 29.5 in the back right and 29 on the front right. I went to let the air down to 29 PSI on the front right tire and saw and interesting cap on it (none of the other stems had a cap) which says “Reset TPM! TPM Inside”. Ok, so I let the air down to 29 PSI, replaced the cap and then fired up the car. As mentioned, the TPM warning came on for the right front tire and I reset the system.
Everything was fine and thankfully no need to replace a $400 tire. Whew.
But, the experience got me thinking and I had not read much about the TPM system, so I thought it was time to learn. Also, I had no idea what the recommended tire pressures were for the vehicle – probably a good thing to know rather than trusting the dealership to know what was best for the vehicle.
I knew that there were two places where I could find the tire pressure recommendation for my vehicle. The first was on the inside door of the vehicle (most cars have this) and the other would be in the manual. I decided to check them out and see which I should use. What I found was a little interesting.
The inside door of the vehicle had two rating. The standard rating for the front tires (245/35ZR19/RF 93Y) was 35 PSI as well as the rears (265/35ZR19/RF 98Y). But then there was another sticker there that had a rating that specifically stated for driving over 100 MPH, the fronts should be at 39 PSI and the rears at 44 PSI. I found it interesting that there was a specific recommendation for high speeds.
I then consulted the manual and for the 19” wheels, it recommended 35 PSI for the front and 36 PSI for the rears. I went ahead and adjusted the pressure to the manual recommendations of 35 PSI and 36 PSI, and then I reset the TPM.
Continuing on with the TPM – I learned a couple of interesting things about BMW’s TPM system for the E90 M3. Generally speaking, it is not made to detect slow or small leaks, but general or significant loss in pressure that takes it below a certain threshold. For example, in the iDrive system – I cannot tell the actual pressure – I am only notified when it drops below a certain threshold.
In addition, cold or freezing temperatures may cause a false alarm on the tires. In my case, this occurred during a recent bout of cold weather here in Southern California with the temperatures dropping below 40 degrees. With every 10 degrees in ambient temperature change, there is a difference of 1.5 PSI within the tire. That means that a 30 degree drop in temperature would lower the PSI in the tires by 4.5 – in some cases enough to trigger the TPM warning light.
Lastly, the recommendation is to check / add pressure to your tires at a “room” temperature in the 60-70 degree range after the car has been sitting for awhile. Doing this outside of this may trigger a warning as the tires may be warmer from being driven and/or it is significantly hotter or colder than the “room temperature”.
I have definitely learned a lot about the BMW TPM system through this – perhaps you have as well!