zathras

Got the low pressure warning on my rr tire today. The warning fluctuated between -6 and -2 when it got warmed up. However the TPMS indicates pretty much the same pressure all around as I've been running for the past few weeks.

I stopped for air but first verified the tpms was reading the same as my digital hand gauge for each tire. So I went to add a couple lbs all around, but I couldn't get the rr to go above 35. Basically I could hear almost no air going into the tire. It was 34.5 when I started. The other tires took as much air as I wanted, and I could hear the normal hiss filling them up, so I know the air pump was working fine.

Any thoughts on what this might be? Bad valve stem? Bad TPMS unit? Tire is visibly fine, and the ride seems normal.

MJBird993

Odd that you couldn't get any air into the tire. Sounds like the valve itself sits down just a little bit lower than the pump would like. I would try a different adapter (assuming that you can replace it). If you can't, find a friend with a compressor.

Or maybe you're just not pressing down hard enough.

As for the original TPMS message, I got one a week or two ago when the temps went down. The tire wasn't really low, but for a brief moment the car thought it was and set the light. Then it (the warning) never went out, even though the pressure (after being warmed up) came back. I pumped in some air anyway, and even after multiple driving cycles, it wouldn't stop telling me that the tire was low. I don't really understand the system, and the manual was no help. I ended up changing it to comfort settings and back again, and that "reset" the system. Crazy Germans.

inspired626

It's probably just the valve stem. I'd take it to a tire shop to see if the issue is indeed that.

plenum

Couple of thoughts -- First, how are you checking the pressure? Handheld gauge, or looking at the readout in the dash display? Use a handheld if you aren't.... You want to check pressure at the valve.

Second, the only reasons you would not be able to get more air into a tire is if the valve itself is faulty, or the tire is "full". Sounds to me like you have a valve stem core problem....

Oh, and you didn't have Nitrogen did you? Not that it makes much difference in your case of not being able to add pressure to the tire, just curious.

zathras

I do have a digital hand gauge that I used to verify the TPMS readings.
It was just compressed air as far as I know- this was at a free air station and I'm sure they wouldn't be giving out nitrogen for free.

So this morning it was giving me the -4 warning with 32 showing on TPMS. digital hand gauge showed 32.5. So I stopped again just to see if I could get it filled up any at all, and suddenly it took all the air I wanted to give it just fine. Got it up to 36 to match the LR and by the time I got to work they were both at 38 on the TPMS... so I'm guessing something in the valve stem was just sticking yesterday. very strange. But the warning light is gone so all's well that ends well... I think I'll test putting some air in it a few times over the next few weeks. If I don't have any more issues I'll just chalk it up to a random glitch, but if it has a problem then I'll probably take it to get the valve replaced.


Here's another question since we're on the subject...
I've noticed that the TPMS numbers don't change equally between the cold and hot readings. eg 32/32 34/34 cold becomes 35/35 38/37 hot. so one tire is increasing more when the tires are hot. sometimes it's as much as 2 or 3psi, and sometimes zero.. but usually 1psi diff on one tire. Is this just the nature of these things, that they're not that precise? Or is one filled with nitrogen perhaps?

Macster

The problem you had adding air is not a random glitch. The tire valve stem valve is bad. My advice is to get the car to a proper tire shop and have the valve stem/valve or just the valve replaced.

What will happen at some point is you will add air to that tire and the valve will develop a slow leak and the tire will go low as you drive. Since you have TPMS you'll get an early warning -- if you have not decided to ignore the TPMS -- but best to avoid this at all.

Get the valve stem/valve replaced.

plenum

Ahh, the old "sticky valve" problem... Ok that is good to hear as it is just a blip on the radar and nothing more serious.

As for your last question - Can't say I know. There are so many things that might cause variations from tire to tire, including air currents, exposure, and so on. The basic rule of thumb for tire pressure is that all gasses expand when heated and contract when cooled, tire inflation pressures rise and fall with changes in temperature by about one psi (pound per square inch) for every 10° Fahrenheit change in temperature. Ambient temperatures, the sun's radiant heat, or the heat generated by driving causes the tire pressure to rise. I don't think Nitrogen reacts any differently, but will remain more consistent throughout the range over time (you will loose less Nitrogeon molecules through the tire rubber than you would regular air). If you Google Nitrogen vs Regular Air in tires you will find tons of useful details on this topic....

Good Luck!

Noah Fect

If that's true, wouldn't you end up with a tire full of nitrogen anyway on a long enough timeline?

plenum

Not really.... I mean, if what you are thinking is that if you just wait long enough, the Oxygen (and other gases) will filter out through the tire rubber, and leave behind pure Nitrogen molecules you might be waiting for a long time... "Tires are typically inflated with air that’s a combination of roughly 78% nitrogen (N2), 21% oxygen (O2) and 1% miscellaneous gases." It is rare that you would ever have a pure N2 situation, even after multiple cycles or purging the air from the tire and refilling with N2. Tire Rack research stated that the highest purity achieved in most cases was 92-94% or something like that.

Larry Cable

could be a couple of things:

1) margin of error in the TPMS
2) relative humidty of the air in the tires
3) you are turning left or right more than the other direction... hence the loaded tires warmup more than the unloaded.

MarcusG

If your tire wont take any more air then your air compressor doesn't have enough *****. With a big boy air compressor you can explode your tire completely if you wanted to.

As for the different readings on you sensors, this is common. Frustrating for sure.

If it were me, I would remove all of the air/nitrogen in all four tires and start over cold. You can easily own 4 different tire air pressure gauges and get four different readings. I do. I thought I had one that was fairly accurate but it turns out it wasn't. Now I just average the two that are closest that I know to be almost accurate and unbelievably enough they are pretty close to what the car tell me.

WCE

One or two psi difference in street tires driven on the street is basically not relevant.
My new BMW has 1/10 psi resolution (ridiculous) and can be programmed to display temperature also (potentially interesting).

chuck911

Good example here of why cold pressure doesn't matter, its hot (working) pressure we care about: the cold 36 went to 38 just driving to work. At about 40 tires start getting hard enough to really start losing grip, which warm day/hard driving could happen real quick.

Any digital guage, TPMS or your hand-held, necessarily has to round off. Depending on how that works a .01 psi difference could show up as 1 psi. This is one of those situations where a $20 analog dial outperforms. What you see is what you get, no rounding error at all.

Nitrogen is mostly what you get whether or not you're using "nitrogen". At least so long as your travels are restricted to Earth.

mtbscott

My TPMS readouts are pretty much identical to the analog air gauge I use at home. And yes, it is perfectly normal for all four tires to be lower when the ambient temperature drops much below 65 or so, but they warm up a few psi as soon as you start driving.
It's also normal for a pound or two variation as conditions change. For instance, when I go out to lunch in my car and it's been sitting in the sun facing south, it's not unusual for the tires on the drivers side (in the direct sun) to be a bit higher than those on the right, as soon as I've driven a mile or two, they're the same again.

The absolute WORST TPMS sensors I've encountered on any car I've owned are on MINI's. It's only dropping to around 63 at night here right now, and every morning this week I've gotten a low tire warning (MINI doesn't have psi readout) and have to reset it, even though my tires' pressures are all correct with a gauge. Too sensitive, and it's been the same way on the other four MINI's that I've owned.

VintageRacer

The car's computer never reads the same as my gauge (racing gauge to 1/4 pound). I can add 2 pounds to the tire, and it will not register on the computer.

Whatever.