Dear Tom and Ray:

What is a throttle body, and why do I need one? I took my 1993 Mazda 626 for its annual state emissions inspection, but the garage couldn't perform the diagnostics because the car was idling too fast. I then took my car to the dealer and had them check out the idle. They told me that my throttle body was shot, but they could get me a new one for $1,000. However, they were able to adjust the idle so I could pass the emissions test. My car seems to be running fine. So, I ask you again, what is a throttle body, and why do I need one? -- Cathy

Ray: The throttle body is the throat through which the air passes when you step on the gas pedal. And it costs $1,000 because it's a finely crafted and machined piece of. . . well, cheap aluminum.

Tom: What happens to them is that the throttle plate, the steel "flap" that lets air in, wears a groove into its aluminum housing or "body." And when that happens, too much air is allowed to get through, even when the throttle plate is closed. So it's as if you have your foot lightly on the gas pedal, even when you don't.

Ray: What the dealer did was adjust the air bleed screw, which cuts down the amount of air getting through to the cylinders at idle. That returned your mixture to a more normal setting and allowed you to pass the emissions test.

Tom: But eventually this will happen again. And there's only so much adjustment in the air bleed screw. Eventually they won't be able to turn it anymore, and then you'll need a new throttle body.

Ray: So ask your mechanic how many more turns you've got left on that air bleed screw, and that'll tell you how much time you have -- to either save up for a new throttle body or unload this car on a co-worker you don't like.

In their pamphlet "Should I Buy, Lease, or Steal My Next Car?" Tom and Ray break down the strategies for buying a car, so you can make the most of your money. 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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