Helpful procedures for chasing down annoying engine, AWD drivetrain, and wheel vibrations.

Vibrations were an accepted part of motoring in the early days of automobiles. Hard rubber tires mounted on rickety wooden wheels clattered down cobblestone streets and rutted, rock-strewn roads. The bone-jarring bumps and thumps transferred to the passengers’ rumps through crude suspension and primitive springs. The rumbling engines also provided a less-than-enjoyable massage to the posteriors of those who chose to endure a trip aboard an early 1900s motorcar.

One hundred years later, modern motorists have come to enjoy the blessings of quiet, comfortable driving. Now, when vibration occurs, it is a sign of trouble. Over the last few decades, advanced engineering has created smoother-operating mechanical components. The elimination of noise, vibration and harshness — known as NVH to engineers — has been a priority for customer satisfaction, vehicle safety and durability.

Still, annoying vibrations may occur in the engine, drivetrain and wheel systems. These pesky “vibes” are an indication of a mechanical malfunction. If not repaired, your customer will certainly not be happy with the vibration and noise, plus the vehicle may end up with a more severe problem.

A Few Things to Remember:

• Any part or component of the vehicle that spins or moves can create vibration. Lack of lubrication, wear or damage are usually the most common culprits here.
• Any part that is loose from its normally fixed mounting can cause vibration. A part that is supposed to be solidly in place can cause noise if it becomes loose.
• Vibration usually creates noise. The friction of two surfaces harshly rubbing against each other when they are not supposed to will normally result in a sound emanating from the agitated source. A loose part or component can create a harmonic vibration and resulting sound.
• Vibration can be happening in one area of the vehicle, yet seem to be coming from another area. Listen from both the inside and outside of the vehicle. If not sure, ask one of the other technicians in your shop to listen also.

Based on these factors, you can approach a vibration complaint with logical steps. Covering all the possibilities that could cause vibrations in all Subaru vehicles would be difficult. However, there are a few guidelines you can follow to help pinpoint the source of the vibration.

Diagnosing Vibrations

Electronic vibration finders and analyzers are available, either in handheld versions (such as the popular Chassis Ear), or as accessories for other diagnostic tools, but can be quite costly. Most repair facilities have at least one automotive stethoscope around. This is an inexpensive device and can be very helpful.

As a preliminary, be sure to check for any diagnostic trouble codes (DTCs) stored in the

vehicle’s system. If the source of the vibration is not quickly discovered, testdrive the vehicle. Do not be totally convinced that what the customer has perceived to be the exact type of vibration, or where it is emanating from, is actually correct. In fact, it probably isn’t. Your experience and training may reveal the real problem.

Engine Shakes

First, take comfort in knowing that Subaru horizontally-opposed “boxer” engines were designed from the ground up to avoid potential vibration. This balanced, even-firing designassures smooth crankshaft revolution that helps avoid vibration problems and results in outstanding engine durability and an extremely low level of NVH.

Traditionally, one of the main sources of engine vibration has been the crankshaft — the central spine of engine operation. Crankshafts rely on oil-bathed bearings to eliminate friction. Older inline and V-configurations used longer crankshafts to accommodate the piston placement and therefore needed more bearings to support the crankshaft. Longer length and the number of bearings required for this length increase the possibility of bearing wear and failure. It used to be commonplace to replace the bearings — and perhaps the crankshaft itself — during an engine overhaul.

The crankshaft in a Subaru engine is short because a 4- or 6-cylinder horizontally-opposed engine has two banks of two or three cylinders rather than a long single row of four or six cylinders, as with in-line or V-block engines. Because the horizontally-opposed engine is arranged symmetrically, the weight is more balanced and the crankshaft does not require extra weight for counterbalance. These two features make the crankshaft both short and light, a significant advantage for any engine part that revolves at such high speeds. The crankshaft is forged from extra-hard chrome molybdenum steel, and the engine is designed with five main bearings to support the crankshaft. This combination produces a strong, durable engine.

If a Subaru vehicle arrives at your shop experiencing engine vibrations, ask the customer if any engine work has been done, recent or not. Any replaced internal engine parts that were not specifically designed for that particular engine may over time create an imbalance or vibration problem and it may not show up immediately. That is why using the correct O.E. part is so important in engine repair. This is even more important if engine performance work, such as the installation of performance pistons, has been done to the vehicle.

Damaged or worn engine mounts can be a mystifying source of vibration. The vibration may seem to be emanating from under the vehicle and be mistakenly diagnosed as suspension, chassis, brake or wheel problems. On high-mileage vehicles, or if the vehicle has been in an accident, be sure to inspect the engine mounts for damage or wear.

The chart on page 16 can be helpful in locating the source of engine noise and vibration.

AWD Drivetrain

Drivetrain vibration can be especially difficult to diagnose. Many times, the problem only appears at cruising speeds of 40 mph or greater. Vibration while cruising may be caused by many things, including propeller shaft problems, front or rear drive shaft maladies, hub unit wear or damage, an unbalanced tire, improper tire inflation pressure, improper wheel alignment, or even aftermarket wheels.

The chart to the right can help you diagnose drivetrain vibration problems.

Brakes

Vibration and the accompanying noise in the braking system can be a source of customer complaint. The customer may feel that the noise indicates an unsafe braking condition. To maintain your relationship with the customer, always consider the problem as serious and take the time to explain the repair you performed. While brake squeal and pedal pulsation are not as prominent as they were some years ago, they still occur, and fixing them right the first time can make you a hero to your customers. The chart to the right below may be helpful in locating and correcting brake vibration and noise.

Wheels and Tires

A wheel/tire assembly that is out of balance will usually cause a wobble or shake that starts at low to medium speed and gets progressively worse as the speed increases, depending on how serious the imbalance is. The cause can be the tire, the wheel, a lost weight, or any combination. It can also be caused by loose lug nuts or improperly inflated tires. If you find a wheel and tire out of balance, check the other tires and re-balance them as needed. Follow all mounting and tightening procedures for the vehicle. While you are checking the wheels, be sure to inspect the hub, bearings and suspension. Finally, check the alignment of the vehicle and correct any mis-alignment.

Be sure to consult the correct Subaru Service Manual. It’s available from your Subaru N.E.W. Horizons Dealer, or on the Subaru Technical Information System website at http://techinfo.subaru.com.

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