Dear Tom and Ray:

I have a 1987 Honda Accord with almost 101,000 miles on it. There's a problem that my mechanic can't seem to fix. The car vibrates at stop lights, so I put it into Neutral which seems to help some. But it jerks really hard when I put it back into Drive. I know I'm not giving you much information, but do you know what might cause this?
Beth

TOM: Sure, Beth. Information doesn't help us anyway. You should know that by now. We rely heavily on conjecture.

RAY: My conjecture is that your idle speed is set too high to mask some other problem. That's why it jerks so hard when you put it into Drive.

TOM: Your mechanic probably tried reducing the idle speed to the proper setting, but when he did that, he found that the engine shook like Jerry Lee Lewis after four cups of Starbucks dark roast.

RAY: So then he did what any stumped mechanic (or any mechanic with a cheapskate customer) would do. He boosted the idle speed back up to mask the problem!

TOM: So the first thing you should do is a compression test. With 100K on the car, your valves may be shot, and that would cause rough idle.

RAY: If the compression is OK, and this is an Accord LX (carbureted), my guess is that you have a vacuum leak which is causing one or more of your cylinders to misfire at low engine speed. We call that a "lean miss at idle."

TOM: Not to be mistaken for one of those teenage girls who hangs out all day at the mall.

RAY: If you want this problem solved, you're going to have to find out why the engine is running roughly at it's normal idle speed. Then you can have the idle speed lowered back to it's original setting, and go back to having a small jerk when you put the car in Drive...like most other Accord owners.