What art remains in the field of modern auto repair has less to do with the repair itself than diagnosing the problem beforehand. With all due respect to mechanics who specialize in just replacing stuff: any monkey can throw parts at a car until it stumbles across the broken part and replaces it. When you're talking about saving dollars, the smart money is on diagnosing problems to the finest possible point before turning a single wrench. While onboard diagnostic systems are pretty capable, at some point you're going to have to compare what you find while pulling codes with what you observe with your own two eyes. Or feel with your hindquarters, whichever applies best. With that in mind, it might help to have some sort of guide or chart to refer to help you track down symptoms.
Diagnosing your vehicle problems is wise to avoid accidents
Vibrations by nature are a result of cyclic imbalances; in other words, something spinning that's heavier on one side than it is on the other, or something that exerts more force in one direction than it does the other. There are really only a few things moving when your car isn't, and they have to do with the engine and transmission. Vibrations at idle are often the result of a single- or multiple-cylinder misfire, which causes the engine to shake. The harmonic balancer is a kind of rubber puck that sits on the front of your engine's crankshaft and absorbs normal vibrations from the crankshaft. When it fails, you can expect engine vibration at idle. Vibration can also come from engine contact with the body through the exhaust system, and can also happen through the transmission or via a bad engine mount.
Transmission problems can also cause vibrations at idle because the transmission turns at the same speed as the engine. A malfunctioning or broken flywheel, flexplate – the disc that connects the crankshaft to the torque converter – pilot bearing, clutch pressure plate, torque converter or transmission input shaft can all cause vibrations at idle. Vibrations that get significantly worse when you put the car in gear with your foot on the brake are usually either the result of a misfire, a leak in one of the engine's vacuum tubes or a torque converter problem.
That increase with engine rpm
You have to draw a distinction between vibrations that increase with engine rpm, and those that increase linearly with speed. Oddly enough, many engine vibration problems smooth out at speed, when crankshaft inertia keeps the rpm more stable and the misfires happen quickly enough that you might not notice them. Vibrations that get worse with rpm usually indicate either an ignition system problem or an imbalance with the engine as a result of a malfunctioning harmonic balancer, flywheel or flexplate.
That increase with road speed
These kinds of vibrations usually result from either an imbalance in the drivetrain or a tire problem. Drivetrain vibrations may be due to transmission output shaft issues, a bent or dented driveshaft or front-wheel-drive axle shaft, a damaged U-joint or missing wheel balance weights. The latter is actually quite common, because wheel weights can and do fall off. Vibrations due to weight oscillation are an odd thing; they can come and go at different speeds, rising in severity from 10 mph to 60 mph, disappearing from 60 to 65 mph, and then coming back with a vengeance. Vibrations resulting from loose or damaged steering and suspension components typically mimic the effects of drivetrain imbalances, since the suspension tends to wobble cyclically – like imbalanced drivetrain parts.
Tire-related vibrations, though, almost always start at low speed and get worse very quickly with road speed. Tire bulges and flat spots, for example, will create a sort of slow wobble at 10 mph, and will increase in strength and speed with road speed until they turn into a jack-hammering pummel to the kidneys at 70 mph. And they'll keep doing it until impact on the road hammers the tire to pieces or overheats part of the tread and causes it to blow out. Loose lug nuts, and in some cases, warped brake rotors, can do the same thing. Warped brake rotors will usually only manifest under braking, but can, under certain circumstances, cause an oscillation vibration at road speed.
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