So you're driving along and suddenly you feel a vibration coming through the steering wheel. Or perhaps you're feeling it coming through the floorboards and under your seat. Maybe you don't really feel it all, but you hear it. The only thing worse than driving a car with a vibration, is trying to find where the bad vibes are coming from. Everybody eventually comes across this problem, and rarely if ever is the problem found through simple diagnosis. In a commuter car the problem is simply annoying to the driver and passengers. In a built up street/strip car, the bad vibe can be an indication of a component about to fail. In both cases if the vibe is not discovered and taken care of, there could result serious damage to the engine or drive train.

Having been through our fair share of hair pulling episodes trying to isolate shakes and rattles, we decided a "how to" diagnosis article would be useful for our readers. Of course not all of you are experiencing this problem right now, so skim through this page and bookmark it. After all it is articles like these that really prove to be a invaluable resource when trouble strikes.

Vibrations can be separated into there categories; engine related, driveline related, and none of the above. Engine related vibrations are typically caused by problems with the balance of the internal rotating assembly, however they can also be due to other mechanical or electrical factors affecting the engine. To isolate whether the vibe is engine related, the simplest test is to place the transmission in neutral and with the car not moving; check to see if the vibration can be felt at idle, as well as throughout the rpm range. If there is no vibration present, you know that your problem is not related to the engine. If you do feel a vibration, it is most likely engine related.

Driveline vibrations are generally caused by anything from the transmission to the axle assembly, and the driveshaft in between. Driveline vibrations
can only be felt when the car or rear wheels are in motion. Typically if the vibration occurs in a specific rpm range, it is driveline related. If it occurs at a specific speed, suspect the wheels.

Finally there are types of vibrations that don't fall into any of those categories; in fact they can be
vibrations due to normal engine harmonics.

Once you've determined the general category of the vibration, the next step is to systematically go through the usual trouble areas as listed below.


External engine: Worn or separated motor or transmission mounts can lead to some funky vibrations. Broken mounts are hard to detect without getting under up close and really inspecting the rubber section of the mounts. Also check the rubber layer sandwiched in the harmonic balancer. Many times the rubber will separate allowing the outer section of the damper, which contains the counter weight, to move and change the engine balance.

Take a very close look at transmission and motor mounts. The rubber can separate, but with the weight of the engine or transmission compressing the mount, you'd never detect it at a glance.

Internal Engine: This category primarily deals with engine balance. If you just built up a new motor, or bought a used one, and it has had a vibration from the minute you first fired it up, chances are the problem is in the balancing of the engine. Ford engines have used a variety of balance factors over the years. All small block Fords, including the 351C and 400M are externally balanced. However the early 221-302's used a 28oz. counterweight factor while recent years all use a 50oz. imbalance. This means you must use the same crank, harmonic balancer and flywheel or flexplate. The Ford FE's, 429 and 460 engines are all internally balanced, except, for the 410 and 428. Also the Ford truck series (FT) are all externally balanced, so keep this in mind if you are using the heavy duty forged steel cranks from the FT in your FE buildups. Using mismatched harmonic balancers, cranks, or flywheels is guaranteed to cause vibration problems. There is no two ways about it, it will occur, and it will be very noticeable. As a matter of fact, it will result in destroyed bearings in short order.

Small block Ford harmonic balancers and flywheels used 28oz counterweights in the early years then switched to 50oz. Mixing the two varieties will cause serious engine balance problems.

However just because you use the correctly matched crank, balancer and flywheel, does not ensure a smooth spinning engine. New pistons, rods and even clutch pressure plates also effect the balance. Thus it is a good idea to always include a balance job when building or rebuilding an engine. Most good machine shops can take mismatched crank, flywheel and balancer and adjust them to achieve the proper balance. This means that if you need to use a four bolt 50oz damper on your 289, your machinist can make the proper weight adjustments to make this possible.

Most Ford flywheels/flexplates are indexed to the crank; meaning they only bolt on one way in order to properly phase the counterweights. While it may look like the mounting holes in the flywheel are evenly spaced, they are not. Many times inexperienced individuals, thinking their flywheel was drilled incorrectly, try to solve the problem by enlarging or egging out the holes. This will result in the counterweights being in the wrong position, and a horrendous vibration. If you've bought a used engine or flywheel, and can't find the cause of a vibration, check this out.

Ring gear separation. This is rare, but it has happened to us. The tack welds holding the ring gear to the flywheel came apart. This in itself may not cause a vibration, but in our case it led to improper phasing of the clutch pressure plate. Good machine shops ask for the clutch pressure plate, so they can balance it along with the rest of the rotating assembly. They usually stamp an indexing mark on the ring gear and the pressure plate so you can line them up properly. In our case, since the ring gear had come loose, the index mark had moved, so when we bolted up the pressure plate it was actually in the wrong position. It took five weeks and as many tranny removal/installs to find this vibration!

Torque convertors are also neutrally balanced. While it is very rare for a torque convertor to cause an out of balance, a convertor that has separated or the internals have come apart can certainly create a vibration or rattle. Usually this is accompanied with a poor performing and slipping transmission.

A very common cause of vibrations is a misfiring cylinder. Of course you'll be able to tell this apart from a mechanical problem due to the performance loss, but nevertheless it can and has been overlooked. A cracked spark plug insulator, smashed gap or broken ground strap, are some of the causes. If you suspect a misfiring cylinder, the best was to isolate it is to pull each spark plug wire one by one, at idle, and notice the resulting rpm drop. A misfiring cylinder will not result in an rpm drop.


Transmission: There is not much within a transmission itself, manual or automatic, that will cause a vibration without any other symptoms, such as poor shifting or slipping. However, as mentioned above, a pressure plate that is out of balance can cause a vibration. Pressure plates are supposed to be neutrally balanced from the manufacturer, but they rarely are exactly zero. If you experience a vibration after installing a new clutch, the pressure plate could be the problem. Have it matched balanced to the old one.

Other transmission areas to check:
Loose or misaligned bellhousing.
Worn or missing pilot bushing/bearing.
Worn tailshaft bushing.

Driveshaft, Rearend gears, and pinion angles: Driveshafts also need to be balanced. However if the driveshaft is damaged, or bent, it wont spin true and result in a vibration. Most factory driveshafts are "phased" with the rear pinion yoke. Usually there is yellow or other colored paint marks on the driveshaft and pinion yoke to designate the phasing. If you've had the driveshaft out recently, try reinstalling it 180 deg. over and see if it clears up the vibration. Worn u-joints will certainly cause a vibration and "clunking" upon braking or acceleration. Check them periodically.

While rearend gear problems don't typically manifest themselves in a vibration, gear ratio changes do contribute to certain types of driveline vibrations. Actually the vibrations are a result of the change in speed of the driveshaft as a result of a gear ratio change. The length, diameter, composition, and angle of a driveshaft, as well as the final gear ratio, determine what rpm (technically called the "half critical" speed) at which the driveshaft will experience a natural vibration. (Incidentally, critical speed, is a physical term for the speed at which a rotating shaft begins to wobble and become unstable As you can imagine, this can be a very dangerous condition for a driveshaft. Fortunately for most driveshafts, this does not occur until well over 10,000 rpm.) All driveshafts have a certain level of vibration, but with the typical factory gear ratios of 3.27 and lower, the vibration may not occur until above 5000 engine rpm - well beyond the typical sustained driving rpm. Numerically higher gear ratios will bring this range lower. So if you install a set of 4.56 gears and suddenly feel a vibration while cruising on the freeway at 3000 rpm, it is most certainly due to the gearset lowering the the half-critcial speed of the driveshaft. Aluminum driveshafts do a better job at dampening vibrations than their steel counterparts. If you've isolated a particular vibration as being due to a gear ratio change, an aluminum driveshaft may help out.

Excessively lowered (or raised) vehicles are prone to driveline vibrations due to the severe angles placed on the driveshaft and u-joints. The ideal angle of the driveshaft should be between 0 and 3 degrees sloping downwards from the transmission to the rearend. To measure the driveline angle, use a degree finder to measure the angle of the driveshaft (wheels must be on the ground or loaded on a lift.) Then measure the angles of the front slip yoke, and the rear pinion yoke and determine the average of the two. Subtract the driveshaft angle from the average of the two yokes and that is the static operating angle of the driveshaft. If you suspect a driveshaft angle problem as the cause of the vibration, find a good driveline shop that can help you properly setup the correct geometry.

Improper driveshaft angles can result in vibrations and accelerated u-joint wear. For engines operating at 5000+ rpm, the driveshaft should not be at more than a 3 degree angle.
[Image from http://www.iedls.com/guide.html ]

Other Causes

If you can't pinpoint the vibration in the engine or drivetrain, start looking elsewhere. Listed below are some other elusive, but common, causes of vibrations.

Wheels and Tires: Everyone has at one point or another experienced an out of balance wheel. The vibration is occurs at a very specific vehicle speed, rather than engine rpm, and can be felt in the steering wheel or the seat. The easiest way to isolate an out of balance wheel is to rotate the suspect wheel and see if the location of the vibration changes. If you feel it in the steering wheel, move the front wheels to the back, and see if it is now felt in in the seat or under your feet. Of course if you just got a new set of tires and neglected to balance the wheels, you may get a vibration from all four wheels. If the vibration appears out of the blue, check the wheels to make sure a balancing weight did not fall of.

Other areas to check:
- Wheels out of alignment.
- Flat spot on tire or radial belt separation.
- Exhaust pipes or header tubes touching the chassis or interfering with steering components.
- Broken or wobbling cooling fan.
- Worn waterpump, alternator, or AC compressor bearings.
- Worn
axle bearings.
- Warped brake rotors or drums.
- Excessive play in ball joints, tie rods, steering box/linkage.
- Loose body panels or window glass.

- Excessive electric fuel pump noise.

Tech Archives Project Cars Readers Cars Feature Cars