Car Talk: Dubious repair suggestions merit a second opinion

Q: I am the original owner of a 2015 Mazda 3 with just under 28,000 miles. During a recent routine service appointment, the following actions were recommended: replace all four tires ($460, not counting labor), throttle body service ($65) and fuel injector service ($130). I've had 10- to 14-year-old cars and never had one needing the last two services. What do you think is going on?

A: What's going on, it sounds like, is that your dealer has a small boat with a payment due. If he had a 38-foot cabin cruiser, he'd be recommending shocks and an exhaust system, too.

The good news: What he's proposing to charge for each of those services is reasonable, assuming he's selling good quality tires. The bad news: Why he's suggesting them is a bit mysterious.

We have a machine in the shop that cleans the throttle body and fuel injectors. It's called the Motor Vac, and in the past we used it a couple of times a week. But gasolines are so clean these days that we now call it the Wallet Vac. I'm not sure I can even remember the last time we used it.

Those services shouldn't be needed unless your car is showing symptoms of dirty fuel injectors: hesitation on acceleration or a check engine light that's storing a code for a fuel system problem. I can't say anything about the tires without examining them. In fairness to the dealer, original-equipment tires on modestly priced cars often are not of top quality, and some do wear out by 30,000 miles.

I'd suggest you ask your friends (or check mechanicsfiles.com) for a recommendation of a reliable independent mechanic and get a second opinion on all three of these services. If a second shop confirms that you need them, then you'll know your dealer is on the up and up, and you can continue to go there with confidence. But if you get an opposite opinion, you might want to rethink that relationship.

Q: I have a 1995 Honda Accord wagon with 128,000 miles and 16-year-old tires. Do I have to worry that my tires are going to split apart one of these days? By the way, I am a member of the Old Ladies Club.

Tires wear out in two ways. One is through use. Every time you drive, your tires give up a tiny bit of their surface. Eventually, they wear down to the point that there's not enough tread left to provide sufficient friction or channel away water. And at that point, it's time for a new set.

But tires also degrade due to exposure to the ozone in the air. Eventually the rubber will dry out and crack. And once a tire loses its pliability, it is a candidate to blow out.

I don't know how many miles you have on your tires, but with 16 years of exposure to the atmosphere, I'm pretty sure they're cooked. If you look at the sidewalls, you'll almost certainly see hundreds of little cracks.

Because of the degradation of the rubber, tire manufacturers recommend that you buy new tires every six years, whether you've worn down the tread or not. Even assuming that the manufacturers are a little overeager to sell new tires, you're still well outside any reasonable life expectancy for a set of tires.