Tire Pressure Loss

I have read many of the comments re tire pressure loss but don’t know if I have a unique problem or not. I own a 2011 GMC Terrain and my tire pressure is monitored through On Star. Every report I receive tells me my pressure is low in all tires - about the same amount, with little variance. Fortunately I own a air compressor and add air - virtually every 10 days or so.
The dealer tells me all tires have small pores which cause leaks and I should go to a tire firm to have nitrogen put in the tires. (He didn’t say it would seal the tires but indicated it would solve my problem?) Almost all of the comments on your site do not speak favorably of this.
Any thoughts?

Tires do leak air…but no where near the degree you’re seeing it. It might drop a pound or 2 every other month.

What could also be happening is the temperature. This time of year temps are dropping…You fill the tires with air at 70…when it’s 40 degrees…the air pressure will have dropped 3 lbs.

Thnx; that was one of the comments from a client in the waiting room when we discussed our problems. In Texas, it has been extreemly hot, so it would seem that the tires should be gaining psi, but not the case. Nonetheless, will continue to moniter.

I would also suggest the low tech approach and check them periodically with a good old fashioned dial type tire pressure gauge. This will rule out any issues relating to sensors and communication within your vehicle and to your email inbox (if that is how OnStar is telling you your tires are low). If your tires are still losing a significant amount of air, they may need to be separated from the rims, the rims cleaned, and bead sealer used with the reinstallation. Alloy wheels will often corrode where the tire seats to the rim, and this corrosion can create a breach through which air can leak. Steel wheels can do this too, but it is much more common on alloy wheels.

I agree with mark about checking the tire pressure on a regular basis with a dial-type tire pressure gauge. However, on a 2011 vehicle, it is very unlikely that corrosion of the wheel rims is an issue. On the other hand, vandalism is a possibility.

Has the OP considered the possibility that somebody is screwing with him by regularly deflating his tires?

Mark9207 & VDCdriver; thnx for your input. To be honest, I seldom checked my tires on a regular basis and I did not have On Star to alert me. I previously had an 2003 Envoy, which I gave to my son and wish I had it back - if for not other reasons- the rear window was separate and able to be lifted without opening the door; and, my golf clubs could be loaded without having to lower the seat backs of the second row since they would not fit crosswise.
Thnx to both of you.

This is not normal. You have leaks. Nitrogen will do nothing here. If you spray soapy water over the tires (especially the bead area and the valve area), you’ll probably find them.

Electronic gauges aren’t always that accurate. You should get a good dial-type gauge (such as Accu-Gage) and use it regularly. The electronic system should just be a backup that alerts you to a major pressure loss.

There were bad batches of valve stems made in an Oriental country. Any respectable shop should be able to tell you where the problem lies.


Those defective Chinese-made Dill-brand valve stems were made…probably about 5 years ago. I doubt that a 2011 model has these valve stems–unless of course our friends in China are up to their usual poor-quality manufacturing practices.

I still suspect valve stems as the likely problem. The nitrogen suggestion by the dealer is somewhere between idiotic and moronic. Air is almost 80% nitrogen.

Valve stems are an inexpensive fix. Buy 4 stems and an extractor tool at a local car parts store. You must have a compressor to fill the tires, though. While you could pay someone to do this for you, it will cost a lot more than the 4 stems, the extractor tool, and the 120V compressor will. And then you will be able to fill your tires with air any Saturday morning you wish to. One more thing: you need a tire pressure gauge. You can get a great dial gauge at Sears for less than $10.

I’m thinking the problem is the gauge being used. What should happen is that the pressure is set above the “alert pressure”, and then the tire (and wheel and valve) will leak down at worse at 1 psi a month. In theory, the alert pressure is many months of leakage lower than the placard pressure (You know about the placard, right?).

You should also be aware that outside temperature will affect this. For every 10°F chnge, the pressure will drop 1 psi. This time of year, it’s possible to lose enough pressure overnight to trigger an alert.

Plus it is possible that the wheels are leaky - not at the tire/wheel interface, but through the alloy it elf.

Lastly, you probably ought to take the vehicle back to the dealer and have them check on this.

In the part of Texas I live in, it was 63 degrees yesterday morning, and hit the mid 90’s. This is different in that it was in the 80’s in the AM, hitting the 100’s. So it is cooling. All my pressures were down yesterday (service day on my truck), but not by the amounts you seem to be losing. For the rest, I’m mostly going to echo the above.

If it’s really leaking, nitrogen won’t fix it.

Get a decent gauge. You can find one a any auto parts store, and the guy/gal behind the counter will be more than happy to help you choose one. Maintaining the correct pressure has many benefits (handling, economy, tire wear), whereas failing to maintain them can hurt a great deal. Make it a part of your Saturday routine or something. We’re talking about less than 10 minutes of your time, and the more efficient you get, the shorter that will be, too.

Once you start maintaining it yourself, you can ignore that section of the OnStar report.

JT, if one does pick up an extracter tool and new valves with the intent of replacing the valves at home, how is one supposed to pop the bead to change the valves?

AME 51025 Quick Change Valve Tool

One learns something new every day.


… and I should go to a tire firm to have nitrogen put in the tires. He didn’t say it would seal the tires but indicated it would solve my problem …

I think the role of nitrogen in tires is misunderstood. Because the rate of diffusion of gases is inversely proportional to the square root of their molecular weights, nitrogen actually diffuses through a rubber tire at a faster rate than oxygen. (Wikipedia: Graham’s law) Assigning a molecular weight of 32 for oxygen and 28 for nitrogen, Graham’s law predicts that nitrogen diffuses (leaks) through runner at a rate 7% faster than oxygen.

I think the reasons for using nitrogen are:

  1. Oxygen tends to oxidize and age rubber as it diffuses through the tire. Nitrogen is inert.
  2. Air, unless it is dried, can contain a large amount of water vapor. This oxygen/water mixture causes additional corrosion of the wheels. Commercial nitrogen is less than 1% water vapor.

For a scientific reason why nitrogen is used, see Ask a Scientist: Inflating Tires with Nitrogen.

With respect, Mechaniker, the only reason for putting nitrogen in street tires is as a sales gimmick. Some stores have also charged for it, which I find appalling.

Air is roughly 77% nitrogen to begin with. And, when “filling” the tire, shops don’t purge the air, they just add the nitrogen to bring it up to pressure. Which means the oxygen, argon, and misc that comprise the other 33% of the filling of the tire is still in there.

In addition, I’ve never seen anything analytical that proves or even suggests that tires with air in them deteriorate at a faster rate than tires with nitrogen in them. Perhaps in a lab it could be demonstrated that

I read the scientific reason, but it does not take into account that we’re not comparing nitrogen with oxygen…we’re comparing nitrogen with 77% nitrogen, 31% oxygen, and 2% argon etc. And the nitrogen we’re “comparing” is only added to the air in the carcass, not replacing it. And, even the article acknoledges that no information exists on how much difference it makes.

Racing tires I cannot comment on, except to say that they operate at far higher temperatures and are far more sensitive to pressure changes, especially running at their edge of adhesion. And that nitrogen will not feed a fire in the pits as oxygen would.

In raod tires it’s a sales gimmick, pure and simple.

That’s a good trick with the rubber valve stems but don’t try that on a late model GM vehicle. I found this picture of the tire pressure sensor on EBAY.

I don’t think a 2011 should need new valve stems but replacing the valve cores would keep someone busy replacing the “summer air”.

I’m afraid mountainbike misunderstood my post. I was simply musing in print that if Graham’s law applied and nitrogen were more permeable in rubber than oxygen, then for what other reasons might nitrogen be beneficial. I was not advocating the use of nitrogen in tires.

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.)

BTW, when air is replaced by nitrogen in tires, the replacement is usually about 95%.

In 2007 Consumer Reports reported on a nitrogen/air loss study on 31 pairs of various tire types. One member of each pair was filled with air and the other member was filled with 95% nitrogen. The tires were pumped to 30 psi and left to stand outside for one year. At the end of the year, the air-filled tires dropped in pressure, on average, 3.5 psi while the nitrogen-filled tires dropped, on average, 2.2 psi. This is not a great difference.

Consumer Reports: Tires - Nitrogen air loss study I hope one does not need a subscription to Consumer Reports to read this article.

Sorry if my original post was unclear.

Thanks for the post Mechaniker. I read your initial and followup posts and their links with interest. You were correct in understanding that my impression of the forst post was that it supported the use of nitrogen, primarily because it doesn;t “age” the rubber and because ot’s free of moisture. I personally don’t know that oxygen ages rubber in any manner and think the moisture=corrosion problem is highly overrated. In over 40 years I’ve never seen a wheel rim corroded inside due to the moisture in its tire-contained air. Frankly, I;ve never had a wheel corrosion problem at all.

The Consumer Reports article does suggest that nitrogen is better retained by tires than oxygen, but, as you said, the difference is insignificant.

The truth is much more simple, it’s a sales gimmick. It’s like whitewall tires. Nice if there’s no additional charge, but not worth a penny functionally.