To all who have offered suggestions, thanks a lot. Don’t know if my problem has been resolved but you have provided me with some options and information I was unaware of. I will continue to seek assistance from the dealer on valve stems, etc.
SFRanger
I would never trust Onstar to tell me about my tire pressure. Buy a good tire gauge.
TPMS=FAIL? The monitors have worked well in my experience. It is in the experience on this board people in general do not check anything until a warning light. Buy a gauge and check pressure is probably -10 on most peoples car thoughts.
“With respect, Mechaniker, the only reason for putting nitrogen in street tires is as a sales gimmick.”
Chances are that is the motivation, but there are some real benefits of nitrogen. However the benefits are so small that they are just not worth the bother.I’d argue that there are no real benefits in street tires. None. Nada. Zip. Not elimination of rim corrosion (not a real pproblem), not thermal expansion coefficient, not nothng. A difference in leakage of a possible 2.2 psi per year is not a real benefit. Unless you like green valvecaps.
We’re talking about the difference here between “pure” nitrogen and 77% nitrogen (air).
As it turns out, from other readings I discovered that Graham’s law does not apply, and oxygen does pass more readily through rubber than nitrogen, but not to the extent which its greater permeability might imply. (The greater permeability of oxygen in rubber is offset by its lower partial pressure within the tire.)
Are you considering that the inner liner of the tire is comprised of butyl rubber, which is impermeable to “air”? This is the same type of rubber they make chemical suits out of for hazmat people. It’s chosen specifically because it does a superior job of not allowing air molecules to permeate. Tire pressure loss is almost always through the bead, valve or wheel porosity.
It sounds like your tires are leaking down too fast. Try this: Remove a tire and do what it takes to get it under calm water with a strong light so you can see well. Watch for obvious air bubbles forming at areas mentioned previously by others. Sometimes an air bubble will form slowly so that you do not see a stream of bubbles but only a bubble slowly forming. Wipe the bubble away and watch to see if it forms again. If you find one, make sure that there are not others.
The last time I found a slow rim leak, it was repaired with a dab of tire patch cement.
For this you may need a bead breaker. Find the leak and possibly a tire shop can go from there if you don’t have a bead breaker and a compressor.
If you have long tire valves mounted at a right angle to the tire, centrifugal force when running could bend the valve stem so bend your tire valve stem a little to make sure that it is not leaking at speed.
Corrosion? Its a 2011, I doubt that there is enough corrosion on the wheels to cause any leakage. Is the TPMS built into the valve stem? Possibly the nut on the stem wasn’t tightened down enough. Otherwise, as anyone check the see if the valve cores are in good and tight?
When you add air, are you checking the pressure before and after with a good dial or digital gauge? Does you TPMS have a readout on the dashboard? The one we had in the company GMC truck had one of these, and it turned out to be pretty accurate.
Make sure you go through the training process, too. From what I remember, there’s a process details in the owners manual to “train” the system. If it’s not communicating properly, there can be all kinds of wierd results.
The tire pressure is monitored through OnStar? Are you certain?
OnStar requires a subscription. The feds require all new cars to have pressure monitoring systems. Requireing a subscription would, I believe, make the car noncompliant with federal requirements.
Chaissos made a good point. Many of these systems require that they be “initialized” to establish the baseline for the computer. Read your owner’s manual carefully. On the other hand, of you’re adding air every 10 days you defintely have a slow leak.
TwinTurbo —
Are you considering that the inner liner of the tire is comprised of butyl rubber, which is impermeable to “air”?
Tire pressure loss is almost always through the bead, valve or wheel porosity.
If you’re not going to do the math, why post claims about pressure loss due to the permeability? Sorry, the onus is on you my friend…
If it was true, I have at least 5 cars that defy your assertion. >10 years sitting on inflated tires and they’re nowhere near showing any signs of being underinflated let alone any being flat. How’s that for empirical evidence?
TwinTurbo —
Tire pressure loss is almost always through the bead, valve or wheel porosity.
From Table I of Liquid & Gas Permeability at Room Temperature, the relative permeability of N2 and O2 in butyl rubber is 50 and approximately 200, respectively. The permeability dimensions are not important here, but they are stated at the bottom of the table.
The ratio of air lost in an air-filled (78% N2, 21% O2) tire to nitrogen loss in a nitrogen-filled (100% N2) tire is
(0.78 * Permeability_of_N2 + 0.21 * Permeability_of_O2) / (1.00 * Permeability_of_N2) or
(0.78 * 50 + 0.21 * 200) / (1.00 * 50) or 1.62
That is, the mass loss by diffusion through the butyl rubber is 1.62 times greater in an air-filled tire than the mass loss in a nitrogen-filled tire. Because pressure loss in this case is nearly proportional to mass loss, the pressure loss in an air-filled tire should be, theoretically, about 1.62 times greater than the pressure loss in a nitrogen-filled tire.
The aforementioned Consumer Reports study showed that, after one year, the air-filled tires dropped in pressure, on average, 3.5 psi while the nitrogen-filled tires dropped, on average, 2.2 psi.
The ratio 3.5 to 2.2 is 1.59, in close agreement with my theoretically derived value of 1.62 above.
That is why I think, under standard operating conditions, most of the normal air loss takes place through the rubber tires themselves, and not through the valve, bead, etc. as you claim.
Of course, if one has an actual air leak, then we are talking about something entirely different.