What is a throttle body and why do I need one?

Dear Car Talk

Dear Car Talk | Aug 01, 1998

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? --

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

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.

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