When I stop at a red light and am using headlights, wipers and AC, my car shakes like it's going to break apart.

Dear Car Talk

Dear Car Talk | Mar 01, 1997

Dear Tom and Ray:

We have a 1988 Corsica four-cylinder with 140,000 miles. When I use the headlights, windshield wipers and air conditioning, and then stop for a red light, the car feels like it's going to shake to pieces. I can stop the vibration by turning off the A/C.
My friendly mechanic just gave me a tune-up, new plugs, wires, etc. This hasn't helped. I turned up the rpm by adjusting the screw on the carburetor. That seems to stop the shaking at idle, but it created a new problem. Now the rpm is so high that the car travels too fast in restricted zones and I have to ride the brakes to keep it under control. What can I do? Is there some way to cut off the A/C automatically when I come to a stop? Or perhaps step up the rpm only when the car is stopped? -- Kathy

TOM: Gee, I'm surprised you were able to adjust the carburetor, Kathy, because this car doesn't have a carburetor! That's a neat trick. I wonder if we could charge our customers for that. Remind me to put that on next week's specials.

RAY: What you did was adjust the idle on the throttle body, Kathy. And that's OK, but as you've noticed, your car is now idling dangerously high, and the speed is hard to control.

TOM: Adjusting the idle WILL solve the problem and stop the car from shaking and feeling like it's about to stall. But as you suggest in your letter, it has to be done automatically, and only when necessary.

RAY: And believe it or not, your engine is designed to do just that! The idle is supposed to automatically adjust on this car through a device called the idle air control. When the computer notices that the idle has dropped (like when you impose a load by turning on the lights or the A/C), it's supposed to signal the idle air control system to let in more air, which in turn adds more fuel and raises the idle back to its normal level.

TOM: Some part of that idle air control system isn't working, Kathy. It could be the control valve itself, a sensor or perhaps even the computer. And that's not surprising for a Corsica with 140,000 miles on it, so I'd take it to a Chevy dealer and get it fixed.

RAY: What's surprising is that everything else IS working! Now, that's impressive!

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