norviewguy

I live in Hampton roads, Virginia. (The Virginia Beach area) so this morning the hawk finally came out and it's 30 degrees out. The reason I brought up where I live is because if anybody knows this area knows Christmas was 80 degrees the day after that can be freezing and they day offer that 80 again. Very unstable here. All my tires were 33psi at around 60 degrees.

So this morning i started "Lexi" and here tpms light came on stating low tire pressure on what I'm certain is the spare. It is highlighted with a square and says the pressure is 30 psi. The other tires were around 28-29psi but after getting to the gym they all warmed up to 32psi except the spare that's stuck at 30.

The light is still on. do I just ignore it and wait until the temperature is warmer for it to go away or should I inflate?

hms09

I would check all with a tire gage and inflate where indicated to 33 PSI. For the spare I think but not sure the spec is 45PSI.

dlbuckls10

I had the same problem and I inflated all my tires to 35 psi.

Rhambler

Why wait, I would just fill it up and rid myself of that warning. You can always deflate again if you notice it getting high in warmer temperatures.

That's why some people go through the expense and trouble of filling up their car tires with nitrogen. "Air" has a lot of water vapor and that is what typically has the most pronounced effect on pressure changes due to temperature.

That "air" in your tires heats up, the molecules heat up and expand, with water vapor expanding the most (although everything "expands"). Conversely, it gets cold, the opposite happens. Water vapor is the culprit.

Nitrogen is more immune to these pressure vs temperature changes and why some people actually pay money to have their tires filled up with nitrogen. Race cars typically use nitrogen and in fact, some cars from the factory have nitrogen in them. Race cars do it so that the rubber to road contact doesn't change during the race as those tires heat up to extreme temperatures. As those tires heat up, it expands and less rubber comes into contact with the ground. Airplane tires also exclusively use nitrogen as it can get to extreme-cold temperatures 35k feet in the air and the last thing you want is that water to freeze and precipitate out.

Of course, your tires have to be completely purged of "air" to benefit from the nitrogen, but if you want to do it, the temperature swings will be much less: you'll maintain that pressure during cold and warm weather.

norviewguy

how expensive is this and where do people commonly get this done? I kinda want to stray away from the dealership here because they are rapists in my book.

jmcraney

At what level are your low-pressure-warning thresholds set to? In other words, what pressure would trigger a low pressure warning? Filling your tires with Nitrogen provides little or no advantage except to the person who sells it to you.

norviewguy

My friend I am new to the car I have no Idea how to set the thresholds. I see the reset button but I have been reluctant to press it until I know what it means, but i think you are onto something

Rhambler

Honestly, I wouldn't even worry about it if I were you. It's not worth the expense and trouble of doing it. I was just providing a comparison and contrast of why these tires expand and contract so much.

Unless you live in some subarctic environment or a desert region where temperature extremes are the norm or truly need that performance, just keeping your tires properly inflated with whatever comes out of that compressor is enough in my opinion.

Here's what I do, top it off until you no longer get a warning. Then, wait for a time where the atmospheric temperature is fairly average, not too cold or not too hot and measure your pressure on cold tires (this assumes you have a compressor at home). Fill it up to whatever is recommended on your door jam, including the spare.

Now if you don't have a home compressor, just wait until your car is completely cold and find a compressor that is as close as possible and drive directly there. Measure and fill accordingly.

norviewguy

got it! so you vote not to mess with the thresholds?

Here is what I'm thinking. I believe the doorjam says 33 psi front and back. I may fill the spare to 33 and the fronts and backs to 32 since its below freezing today and see what happens when times warm up.

Question, when I fill everything up should the light immediately go out?

jmcraney

You are wise to avoid that "reset" button until you develop a good understanding as to what it is for. It's not really a reset button but instead it is a Set button - it sets the warning thresholds through a process that is referred to as "TPMS Benchmarking" in your owners manual. The warning thresholds can be set over a wide range and on a per tire basis to accommodate a wide range of possible requirements. All new cars sold in the US for the past 10 years were required to have a Tire Pressure Monitoring System and they all work much like the system on your car. I suspect that your car was not benchmarked correctly and your low pressure warnings are related to that. The only way to be certain that the benchmarking is correct is to do it yourself and then carefully control the access to the set switch. Service people and owners who are not well informed about benchmarking sometimes feel the need to press that switch with the expectation that they are doing the right thing and they often cause the warning thresholds to be established at a pressure level that will result in lots of false alarms, or compromise your safety.

I think that if you will adhere to the following advice you will have a safe, comfortable and trouble free ride.

If you continue to have a problem or the advice is not clear, don't hesitate to let us know.

What is probably happening to you is that one or more of the pressures being reported by the tire sensors falls below the warning threshold. Normally the lowest pressure in the tires occurs in the morning, before the car is moved and then rises as the car is driven because the friction and flexure of rolling raises the temperature of the tires. The tires have a positive temperature coefficient, meaning that the pressure goes up as the temperature goes up. In the case of keeping the car in a warm, of relatively warmer, garage, the pressure goes down when the car is taken out into very cold ambient conditions. Typical temperature coefficients for tires is up to 1 PSI per 10 deg F, meaning that if you have your car in a 50 deg F garage and you take it out into 20 deg F air you might see the tire pressure drop 3 PSI. You can easily see that this indeed happens by watching the pressures that are reported in the multifunction display section of the dash. And you will probably notice that the pressure of the spare tire drops farther than the pressure of the tires that are rolling because it doesn’t benefit from friction/flexure action. If the tire-pressure/warning-threshold margin is not sufficient then the dropping pressure will cause the TPMS to give a low-pressure warning. For instance if the tire pressure starts off at 33 PSI and the warning-threshold is set to 31 PSI then when you will get a low-pressure warning after you take your car from a relatively warm garage to very cold. The warning is delayed because the pressure doesn’t drop instantly.

For most tires and most driving conditions the car’s manufacturer and the tire’s manufacturer have agreed that a cold inflation pressure and TPMS benchmark pressure of 33 PSI (that’s the pressure of the tires at the typically coldest part of the day, the morning in most instances, the spare is an exception that I will explain later) will provide the best tire performance (tread wear, traction, comfortable ride), greatest protection from underinflated tires and greatest protection from TPMS false alarms.

What so many people don’t understand is how the warning thresholds are established. We are unable to view the warning threshold settings directly so we must either establish them ourselves or have it done by someone we trust to do it correctly and then be vigilant that no one has reached under the dash and pushed that TPMS Set button.

The TPMS warning thresholds are entered indirectly by a process called Benchmarking. This is accomplished by adjusting the inflation of all 5 tires, four rollers and one spare, to the “normal pressure,” 33 PSI in most cases, and pressing the TPMS Set button under the dash. This establishes a warning threshold that is 25% below the normal pressure and in the case of 33 PSI the warning threshold would be at about 25 PSI.

You should only need to benchmark once as the setting will remain until the button is pressed again. The owner’s manual cautions that the benchmark must be redone whenever the battery is disconnected. I am not sure what happens when the battery is disconnected, I suspect that the benchmark is defaulted to 33 PSI but I always redo mine when the battery is disconnected.

Even though the benchmark only needs to be set one time, the tire pressures need periodic attention. The tires are constantly losing air through a process called permeation typically 0.5 PSI per month but up to 1 PSI per month is considered acceptable. You need to watch the pressure on the multifunction display when you first start up in the morning and keep your tire pressures adjusted for 33 PSI. This also requires a seasonal adjustment in that you may have to let some air out when the temperatures suddenly turn warm and you will definitely have to add some air when the temperatures suddenly turn cold.

About the spare: be sure to benchmark it at 33 PSI. I like to inflate my spare to 40 PSI after I benchmark it at 33 PSI. That way I don’t have to give it any attention for permeation or seasonal considerations until its startup pressure falls below 33 PSI.

I hope this helps.

R Z

The past two Decembers my tire light has come on at the sign of cold. Both times, I went to the dealership, they checked for leaks, filled them and off I went. For me it's become the first signs of winter.

DIVER4USN

Charles'-Gay Lussac's Law

v8mac

Gas Law: PV/T=constant.
when temperature changes, tyre pressure responds directly.

Even here in Houston, temperature swings can be great enough to result in 5 psi tyre pressure difference. (without considering leaks, work-heating by driving).
At these or more extreme temperature changes, there is no shortcut - one must add or remove air.

johnnyg66

I reset all my tire pressures a few days ago. Set them all to 34psi and after about 15 miles or so the pressures goes to 37-38.

On the dash the top reading was the spare followed by front left,rear left,front right and than rear right.

The temp for today was 70.