waynedog

I recently had new tires put on at Costco with nitrogen in them. Now that it has got colder (i.e. <30F) in the morning when I get in my car (parked outside) I get the TPMS warning of low tire pressure. Several tires show 28 psi. After I drive for a while, they come up to 32 psi where they should be. This never happened in the past. What is causing this? What can I do to avoid the warnings every time I get in the car when it is cold outside? When I am driving, is there a way to turn off the warning?

rominl

iTrader: (4)

you are supposed to put air in so the pressure reads around 32psi when it's cold (first thing in the morning before you drive)

the warning comes up for a reason, because pressure is too low

Nospinzone

^
What rominl said.

Plus even if you could turn off the warning (which I don't think you can unless you put more air in the tire), what would happen if you picked up a nail and were really losing air pressure?

jmcraney

waynedog,
The advice you've gotten above is very good but something seems to be wrong and is not being considered. The performance of Nitrogen is only marginally different from air and if you have to pay for it, it is a waste of money. It is not wrong to use Nitrogen but the advantage can only be detected with instrumentation so if you have concerns, forget about it. The pressure in your tires is affected by temperature. For example, with your car parked outside, you inflate the tires, using a high resolution high accuracy tire gauge, to 33 PSI on a 70 degree F day and overnight the temperature drops to 30 degrees F then your tires would measure about 29 PSI. And, the next day the temperature returns to 70 degrees, the tires pressure would measure about 33 PSI. Typical temperature coefficient for tires is about 1 PSI / 10 degrees F. The tires get warmer when the car is driven so the pressure goes up from that too and when the car is parked the tires cool and the pressure goes down. All tires behave this way so no concerns about that. The thing that concerns me about what you told us is your low pressure alarm from tires that are reporting 28 PSI. The owner's manual recommends that the TPMS be benchmarked at 33 PSI, and of course the TPMS has the flexibility to let you use any benchmark you want. The point being, if the TPMS was benchmarked for 33 PSI then the alarm thresholds would be 24-25 PSI. Roughly speaking that means the tires that are alarming at 28 PSI are benchmarked above 37 PSI. Warning thresholds are established at 25% below benchmark.

tyrenfroe

You sure about the benchmark vs. the low-pressure alarm point. I BELIEVE mine are benched at 33psi, and I KNOW they alarm at 27psi and below.

waynedog

Is there a way for me to change the warning threshold %?

jmcraney

Yes, do the "benchmark" procedure and be sure to include the spare.

jmcraney

No, I’m not sure. That sort of information is hard to come by and I have not seen it written down anywhere that I can find. The 25% came from a different system and perhaps it is 20% for our cars. And, determining it empirically is difficult for me. When I was trying to determine this I had a tire gauge with 1 PSI resolution but now I have a tire gauge with 0.1 PSI resolution but I haven’t looked at it again. If you have some knowledge about this please share it with us. You have to keep in mind that the tire sensors are probably reporting in much higher resolution than the one digit resolution of the dash display and that might account for 1 PSI of uncertainty. If you are basing your belief that the threshold is 27 PSI based on what you observe on the dash display, then that could be misleading, depending on your technique. For example, if you are basing your determination on observing that the warning is on at 27 PSI and goes away when the display changes to 28, you have to keep in mind that threshold detectors often use a technique called hysteresis to reduce instability for pressures near the threshold. As an example, and only as an example, let’s say the tire pressure is 30 and the threshold is 26 and the tire starts leaking. When the pressure crosses the threshold, 26, then the alarm sounds. In systems employing hysteresis, some feedback is applied when the threshold is crossed and moves the threshold in the opposite direction. So now the threshold has moved to 27 and the display may have to get to 28, depending how things are quantified, for the warning to go away. I have no doubt that the warning threshold can be characterized using a high resolution gauge and making measurements only when the pressure is decreasing but that would be time consuming especially because of the slow sample rate.

I think that being unable to display the warning thresholds is a weakness. We have to accept that the thresholds are set properly but do not have absolute control of setting them. The switch that sets the benchmark is exposed and can be operated by those who don’t understand the consequences. Some people who don’t understand operate the set switch to silence the alarm or believing that they are resetting the system.

An example of how this can go wrong is failing to consider the pressure in the spare tire when operating the set switch. Let’s say the pressure in four of the tires is 33 PSI but the spare is at 40 PSI and the set switch is operated. Then the spare is swapped for one of the other tires and adjusted to 33 PSI. Unless the benchmark is set again then you have created an alarm nuisance.

If the tires are benchmarked too high you create a nuisance and if they are benchmarked too low you compromise safety.

The owner’s manual cautions us to do the benchmark procedure every time the battery is disconnected. I am not sure what happens there, benchmarks may be established when the battery is reconnected or the benchmarks may be defaulted to 33 PSI for all tires or, something else, but I always do it to be sure.

The TPMS is a great safety feature but requires correct benchmarking, routine adding of air to tires to compensate for permeation loss and seasonal tire pressure adjustment if you want trouble free operation, best tire performance and safe operation.

Here are some links to other threads where we have discussed the TPMS:

https://www.clublexus.com/forums/ls4...ng-system.html

https://www.clublexus.com/forums/ls4...-question.html

https://www.clublexus.com/forums/ls4...-question.html

rominl

iTrader: (4)

if your intention is to change the margin or reference point, you are still treading this problem completely wrong

you need to add air to the tires so they rear 32/33psi when it's cold (first thing in the morning. that is the industry reference on tire pressure

and jim is also correct, nitrogen is a waste of money, it's just a gimmick

tyrenfroe

Thanks for your excellent post. Much to consider. Without getting into the subtleties of hysteresis and such, I will attempt to apply some measurements to a testing protocol and see what I can come up with. Do you think there are any significant differences between the 2007 (mine) and later model LS TPMS systems? I wouldn't think so, but I know the older systems had undifferentiated pressure displays compared to the newest ones where tire positions are locked down. Again, thanks for your very well considered and though-provoking post.