Dear Tom and Ray:

I'd like to know something about turbochargers on cars. Specifically, what is a turbocharger? What does it do and how does it do it? Finally, is it an asset or a liability on a used car? -- Peter

Ray: Good questions, Peter. A turbocharger is known in the parts department as a "3BP." That's "triple boat payment," because it usually costs well over $1,000 to replace.

Tom: A turbocharger is basically a high-speed fan driven by the car's own exhaust pressure. When you step hard on the gas, the exhaust spins the fan very quickly (at tens of thousands of rpm) and the fan forces extra air into the cylinders. When you force in more air, you can then burn more gas and end up with ... more power!

Ray: In theory, it's a great idea. Engineers figured out that, for the average driver, there are only a few times each day that you really need a lot of power from your engine -- like when you're getting on a highway or passing a car on a two-lane road. So why lug around a big V8 or even V6 engine, and pay the gas mileage and pollution penalties that go along with that, when 97 percent of the time a smaller engine would be just fine?

Tom: And, the theory goes, to add that extra surge of power when you really need it, you call on the turbocharger.

Ray: The problem in reality is that the people who buy turbocharged cars tend to be testosterone-poisoned males who have the turbo working all day long. And that's hard on a) the turbo, and b) the rest of the car.

Tom: Also, most cars with turbos tell you to let the engine idle for 30 seconds before you shut it off. And most turbo owners do this for the first week or so they own the car and then forget about it. That idling time allows the oil to cool off the turbocharger. If the turbo doesn't cool off, oil can eventually dry out up there and restrict the passageways. That leads to turbo failure, and the aforementioned 3BP.

Ray: So since turbos are rarely used judiciously or properly, we caution against buying one in a used car. Of course, if you find a Mitsubishi 3000GT Turbo for sale by a little old lady, you can take a chance on it. But even then, I'd be willing to bet she's known around the nursing home as Grandma Leadfoot.

Don't get stuck with a lemon. Be an informed shopper. Read Tom and Ray's guide "How to Buy a Great Used Car: Secrets Only Your Mechanic Knows." To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1998 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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