Car Talk

August 1990

Dear Tom and Ray:
Here's one for you! Ever since my Brother-In-Law adjusted the idle speed on my '85 Mazda 626, it just hasn't been the same! Now, when I turn on the air conditioning, the idle revs up and down, up and down, constantly. When I have the air conditioning off, it revs every time I turn the steering wheel at a stop! What can I do? It's dangerous!
Adrian

RAY: First of all, Adrian, we're sending a copy of your letter to the Society for the Defense of In Laws. Everybody wants to blame his or her in-laws for something, and we've had enough of it! Your poor Brother-In-Law had nothing to do with your erratic idle.

TOM: He probably DID clean out all the change from under your seats and spill cream soda all over your carpet, but he didn't mess up the idle.

RAY: Your car has what's called an "idle-up diaphragm." When you use the air conditioner or power steering, you impose a heavy load on the engine. To keep the engine from stalling, vacuum is automatically sent to the idle-up diaphragm to increase the idle speed. When the diaphragm is sucked in by the vacuum, it pulls a rod attached to the throttle, which ups the idle speed.

TOM: Your idle-up diaphragm probably has a small hole in it. So the vacuum can suck it in, but can't hold it in place. And as the diaphragm slips back and forth, the idle speed moves with it.

RAY: You need a new idle-up diaphragm. See if you can get your Brother In Law to pay for it. Tell him you wrote to us and we said the whole thing is his fault.
Beat up old car

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