Q. Could you please tell what causes a shimmy in the steering wheel? When I drive my F150 over 40 to 50 mph, it starts to shimmy.

A. The typical cause of a shimmy or shake is a tire out of balance. The first place to start is to have the balance checked on all four tires. At the same time, the technician can inspect the steering system for wear and check the overall condition of the tires.

Is it a miracle?

Q. I have a comment and a question related to an earlier column. Years ago, we sold hundreds of cases of ADERCO fuel additive. This product was made in Quebec and marketed in 15 countries. I remember their slogan, “One Pill per Fill.” It was a pill dropped in the fuel tank that allowed the engine to achieve better performance and up to 20 percent better mileage. It was compatible with diesel fuel as well. Although it seemed like a “miracle pill,” it actually worked and we were quite successful in selling large quantities. I have no idea if it is still around or not. Some 40 years ago, before we started selling ADERCO, many of my friends and I used to add a half-dozen moth balls to our gas tank every third or fourth refueling. Vehicles started better and ran better, and overall performance improved with noticeable improvement gas consumption. We would use regular, add moth balls and run an 8-cylinder, high-performance engine with no problems. I have no idea how it worked, but it did. Have you heard of this use and the reason it works?

A. This story has been around longer than I have. It easily traces its roots to World War II when gasoline was in short supply. In theory, the chemical in mothballs would break down the surface tension of the poor-quality gasoline and allow for better dispersion, resulting in better performance and fuel economy. This is the same theory as adding acetone to fuel to increase performance. For practical purposes, it doesn’t work. Even conspiracy theorists admit gasoline companies have one goal: to sell product. If Shell, Mobil or some other refiners had a grade of gasoline that could improve fuel economy by 20 percent, they would sell all they could manufacture.

Suspension problem

Q. I have a well-maintained 1994 GMC Suburban 2500 with 62,000 miles. Lately, I’ve noticed the vehicle gives a hard, choppy, bouncy ride over bumps or rough pavement, much like you’d get with over-inflated tires. There is no shimmy or vibration in the steering wheel when this happens, nor does the vehicle pull to one side. The tires are, in fact, properly inflated and show even wear. The shocks are about 8 years old. The vehicle rides smooth as glass on smooth pavement. I haven’t mentioned this to my mechanic yet. I’m wondering what might be going on.

A. The problem may be a combination of worn shocks and springs. Over the years, the springs or torsion bars in your truck may have worn and are not allowing sufficient suspension travel. Every vehicle has a measurement referred to as ride or trim height. A good alignment technician should be able to measure the height to determine the proper course of repair.

Running lean

Q. I have a 1996 Mercury Grand Marquis LS with only 62,000 miles on it. Recently, my “check engine” light came on. A diagnostic test showed the engine to be running lean on the left bank (assumed to mean the left side of the engine as viewed from the driver’s seat). I was advised it could possibly be the O2 sensor. There are many oxygen sensors on this car and I have a few questions. Is there a way to test these oxygen sensors to prevent unnecessary purchase of more than one when only one is at fault (very expensive item)? What exactly is the function of the O2 sensor? If the sensor(s) are not at fault, what other item on the engine could be the probable cause of this problem?

A. The oxygen sensor’s job is to measure the oxygen content in the exhaust system to determine the proper air/fuel mixture. An oxygen sensor can be tested with a good quality volt meter. That said, the problem may not be the oxygen sensor. It could be a vacuum or intake manifold leak causing a lean condition. If the code is for a lean condition on both banks, the common problem with the Grand Marquis and Crown Victoria models is a contaminated mass air-flow sensor. Cleaning it will often solve the problem.

If it sputters, smokes or stalls, John Paul, the AAA Car Doctor{+s}{+m}, has the diagnosis. For an archive of his past prescriptions for car ailments go to: www.projocars.com

projocars@projo.com