Diagnose Car Vibrations

When diagnosing vibrations it is imperative to notice when and where the vibrations occur. Test-drive the vehicle with this in mind. You need to determine whether the vibrations are a tire issue, an engine or drive train problem or exhaust issue. Feel for the vibrations at an idle state and whether they are felt in the steering wheel. Put the car in gear and see if the vibrations lessen or increase. Drive the car at 10 to 20 miles per hour and feel for side to side shifts and vibrations. Check to see if the vibrations get faster as speed increases. Check to see if there are vibrations going over bumps. Drive the car over 45 miles per hour and notice if you feel the vibrations increasing in the steering wheel. Check the appropriate steps that apply to the vibrations felt in your test drive for the proper order of diagnosis.

Instructions

1. Check the front tires for a lateral or side to side vibrations at slow speeds by jacking up each tire individually. Grasp the tire on the top and the bottom and try to rock the tire. If there is play when moving the tire in and out, the bearing--in the case of a rear wheel drive, or the hub bearing in the case of a front wheel drive--is bad and needs to be replaced. Check for significant damage to the tire: see if there are any bulges on the inside or outside of the tire. Look for any deformities as you rotate the tire, such as flat spots, uneven wear or steel protruding out of the tread.

2. Check the front tires for balancing if the vibration is only at speeds in excess of 45 miles per hour. This can only be done at a tire shop with a balancer

3. Check the engine for vibrations. Test for broken motor mounts. Open the hood and start the engine. Have an assistant stand to one side of the fender in a position to see the engine but not in front of the car. Have the assistant tell you what side of the motor lifts as you proceed or if they see the broken mount. Put the emergency brake on. While pressing firmly on the brake pedal, put the car in drive and slightly step on the gas for two seconds (goose the engine to no higher than 2000 RPM). If the mount is bad the engine will lift up or rock. Now do the same with the car in reverse and watch the engine for the same effect. If it rocks the motor mounts are bad and need to be replaced. Replacing bad motor mounts will eliminate the vibration in the steering wheel. If the engine does not respond to these tests then they must be checked for sagging or cracked rubber isolators. To do this you must look at all the mounts for cracked or broken rubber indicating a bad mount. On a front wheel drive there is generally one top mount for the engine (on the side with the belts) supporting the engine and two lower mounts under the engine. One will be low by the radiator and the other under the back toward the firewall. There will also be one on the top of the transmission holding it attached to the frame. A few will have the front mount under the car behind the crankshaft pulley. This would necessitate jacking up the car to see the mount. In the event it is a rear wheel drive, the mounts are always on the lower sides of the block.

4. Check the car for vibrations that are present but do not send vibrations through the steering wheel. Putting the transmission in neutral, leave the door open, and rev the engine up to about 3000 RPM. If the noise and vibration can be heard in the middle of the car it is most likely the shield on the catalytic converter, which often gets bent and contacts either the converter or the body. Simply take a screw driver and pry it away from the spot it is in contact with. Check the rest of the exhaust by shaking the pipe and see if it contacts the body.

5. Check the car for vibrations at all speeds by looking at all the tires for cupping of the tread. This is an unevenness of the tread that you feel as your hand is run across the tread. Look for flat spots on the tires created by a panic stop where the tires were locked up and slid removing rubber on that spot. This will generate vibrations at all speeds.