Dear Car Talk:

I have a 2007 Scion TC with 87,563 miles on it. Recently, it has started to shake while idling at a stoplight -- or anytime I stop the car and leave it in drive. One person advised me to switch out the old spark plugs. I switched them out, but I still have the same problem. Any advice?

-- Ann

Don't switch out the spark plugs, Ann. That was actually a reasonable guess, if not a very scientific one. The person who advised you to change the plugs assumed you had an engine misfire, and guessed that the spark plugs might be the cause. Apparently, they're not.

If you came into the shop, I'd try to approach it more logically. Yes, that's an enormous challenge for me, but I'd try.

If your check engine light was on and was indicating a misfire, I'd start by making sure all four of your cylinders were firing correctly. I'd do that by disabling one spark plug at a time. When you disable a spark plug, you force the engine to run on three cylinders. So if the engine is running properly, each time you knock out a plug, the engine should run much worse -- badly enough that it's immediately obvious. But if you disable a spark plug and nothing changes, then you know that cylinder wasn't contributing in the first place.

Then you just have to figure out why. It could be a bad coil, a faulty fuel injector, a vacuum leak or something worse.

If the check engine light isn't illuminated, and all four cylinders are running well, then I'd suspect a weak or broken motor mount. Motor mounts are the heavy rubber-and-steel contraptions that attach the engine to the frame of the car.

Their job is to both hold the engine in place and isolate the engine's vibrations from the rest of the car. So if one of your engine mounts is broken or badly worn out, you'll feel the normal vibrations of the engine much more strongly. And you'll notice it most at idle, when you don't have all the other wonderful road noises and vibrations to distract you.

So take it to a good mechanic and ask him to diagnose it for you. He'll figure it out.