August 3, 2012

Car Talk

Tires can't self-inflate, so what's going on here?

Syndicated columnists

Dear Tom and Ray:
The tires on my wife's 2005 Infiniti FX35 are about six months old. We had no problems with them, until three weeks ago. When driving down the highway, the tire-pressure warning light came on. I pulled over to see which one had gone flat, but lo and behold, the right front tire was registering 57 psi! I reduced the pressure to 36, continued my drive home, checked the tires the next morning, found everything OK, figured it was just a fluke and forgot about it. Then, a few days ago, the exact same thing happened again. Same car, same tire, same highway. I've taken the car back to the reputable dealer where I purchased the tires. No one has heard of this before, and they can find nothing wrong with the car or the tire. I don't even think they believe me. What do you think, guys? -- Rob

Tom: We think you're a liar, Rob. There's no way a tire can double its own air pressure without human intervention. One of my ex-wives put you up to this, didn't she?

Ray: I agree that a tire can't gain that much air on its own. Now, tire pressure does go up about one pound per square inch (psi) for every 10-degree rise in the tire's temperature. But even that won't explain what happened in your case.

Tom: Think about it -- even if your tire started at 60 degrees in your driveway and went up to 160 on the highway (which is high), that would add only another 10 pounds of pressure -- not 25.

Ray: Nor does it explain why the pressures in the other tires didn't go up by the same amount. They were on the same car, on the same highway.

Tom: I suppose it's possible that if you had a brake caliper that was sticking on that wheel, highway driving could heat up the wheel itself, which would then heat up the tire. So you can ask your repair person to check for signs of a sticky caliper on that wheel. But to get that hot, it would have to be so sticky that it would be almost seized.

Ray: So the most likely explanation is that it was a measurement error. Did you actually use a tire gauge to check all four tires? Or did you just look at the tire-pressure monitor readings on the car's dashboard?

Tom: If you were just reading the pressures from the dashboard monitor, then I suspect that the pressure sensor in your right front wheel is faulty. Each wheel has its own sensor that sends its pressure reading wirelessly to the car's computer. Sometimes those sensors go bad. It may even have been damaged when you had your new tires installed six months ago.

Ray: So, the first thing you should do is spend 10 bucks and get yourself a decent digital tire gauge. Then, next time this happens, get an actual reading from the tire itself. If it's the same as the other tires, then the tire-pressure sensor probably is bad, and for less than a hundred bucks, you could have it replaced.

Tom: If, on the other hand, a reliable tire gauge actually reads 57 pounds of air, then residing in your front tire is a new species of prolific, rubber-eating, flatulence-producing bacteria that has heretofore been undiscovered by science. Make sure they name it after you, Rob.

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)

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