LITTLE ROCK — DEAR TOM AND RAY:
The tires on my wife’s ’05 Infiniti FX35 are about six months old. We had no problems with them, until three weeks ago. When I was 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 bought 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, gas-producing bacteria that has heretofore been undiscovered by science. Make sure they name it after you, Rob.
DEAR TOM AND RAY:
While I was watching a baseball game, an ad on the backstop said to change your shocks at 50,000 miles. I realize this national shock-replacement company is trying to sell shocks. What I want to know: Is 50,000 miles a hard and fast number? If not, how do you know when to change the shocks?
- Ken
RAY:
No, there’s no reason to automatically change your shocks at 50,000 miles. We see lots of cars these days whose shocks last more than100,000 miles. So these guys are just hoping to double their income.
TOM:
If it works, we’re going to try it, too!
RAY:
Even to estimate how long your shocks will last, you have to evaluate them on a car-by-car basis. First, you have to factor in the quality of the shocks that came with the car, which varies. Then you have to consider the type of driving the car is asked to do.
TOM:
If you do a lot of smooth highway driving, the shocks mostly are just sitting there, doing very little. They’ll last a long time when they’re doing nothing.
RAY:
My brother’s hoping for the same result for himself.
TOM:
Whereas if you drive on a lot of unimproved roads with potholes and bumps and lots of dead armadillos, you easily could go through a set of shocks in 50,000 miles.
RAY:
And how does a mechanic know if you need new shocks? He looks for leaks. If a shock is leaking, it’s either worn out or it’s well on its way to being worn out. That’s the definitive sign.
TOM:
There’s also a practical test you can do at home. You get someone big (try a mother-in-law, for instance) to push hard on a corner of the car and get it bouncing down and up, down and up. Then, when it’s at the bottom of its cycle, you let go and see what happens.
RAY:
If the corner of the car comes up and stops, without going down and up again, that shock is good (assuming it hasn’t started leaking). If the car keeps cycling up and down, even a little bit, after you let go of it, the shock is worn out.
TOM:
Shocks don’t really degrade over time. They’re usually either good or bad, so there’s no need to replace them unless they’ve actually stopped working. Good luck, Ken.
Click & Clack (Tom and Ray Magliozzi) dispense advice about cars in Car Talk every Saturday. Email Click & Clack by visiting the Car Talk section of
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