Engine vibration is usually due to one or more of the following conditions:

Because these vibrations are engine-speed related, they are also normally torque sensitive. These vibrations may appear and disappear at different vehicle or road speeds, but will always appear at the same engine speed.

For example, if a customer states that a vibration is present at 40 km/h (25 mph), 64 km/h (40 mph), and again at 104 km/h (65 mph), and that the symptoms of the vibration are similar at all of these speeds, the vibration is probably engine-speed related. Any disturbance or vibration that is present during the following road tests would be considered engine-speed related:

Any vibration that is present during the Neutral coast-down test is not engine-speed related. The engine-related vibrations covered in this diagnosis are engine-speed sensitive only.

Notice: Do not accelerate against the brakes for longer than 10 seconds. Do not overheat the engine or the transmission. Depending on the vehicle design, the engine will only accelerate to a certain point under these conditions. Also, care should be taken during diagnosis as some disturbances may be created during brake torque that normally do not exist.

Notice: Proper flywheel installation requires carefully WALKING the flywheel onto the crankshaft and stagger-tightening the bolts. Proper torque converter to flywheel installation then requires gradually WALKING the torque converter to be flush with the flywheel, prior to installing the bolts. Do not draw the torque converter to the flywheel with the bolts. If these precautions are not followed, warping or bending of the flywheel and/or damage to the transmission torque converter may result.

Engine imbalance is a condition that exists when a component that rotates at crankshaft speed is either unbalanced or has excessive runout. In rare cases, the crankshaft may be unbalanced. Balancing the component or correcting the runout may bring the disturbance to an acceptable level.

Symptoms

1. Perform the Neutral run-up test, noting the RPM at which the vibration is the worst, and the most severe. Refer to Types of Road Test Procedures .
2. Inspect all powertrain mounts. Repair or replace as needed.
3. Inspect for any bindings or ground-outs in the exhaust system.

Important: Steps 4-17 apply only to vehicles equipped with automatic transmission.

4. Mark the location of the torque converter to the flywheel.
5. Disconnect the torque converter from the flywheel and push the converter back from the flywheel.
6. Tie up the converter, away from the flywheel.
7. Perform the Neutral run-up test again. Refer to Types of Road Test Procedures . Observe the flywheel.

•	Inspect for any wobble or lateral runout.

•	If any wobble or lateral runout exists, replace the flywheel.

•	Refer to the appropriate procedures in Engine Mechanical:

•	Engine Flywheel Replacement

•	Engine Flywheel Installation

•	Engine Flywheel Replacement

•	Engine Flywheel Replacement

•	Re-index torque convertor in 3 different positions. If the disturbance still exists, replace the torque convertor. Refer to Transmission Replacement.

8. If the vibration still exists, inspect the propeller shaft runout. Refer to Diagnosing First-Order Driveline Vibration .
9. If the vibration still exists, inspect the harmonic balancer.
10. If the vibration still exists, the problem is related to residual engine imbalance. Refer to Correcting Residual Engine Imbalance below.

1. Install the washers on the harmonic balancer pulley bolts.
2. Replace the bolts with longer bolts of equal hardness as needed.
3. Install the J-nuts on the outer edge of the circular holes in the flywheel.
4. For M/T only, inspect the flywheel and the pressure plate for the following conditions:

•	Correct factory indexing (The white paint spot on the flywheel and the pressure plate should line up.)

•

Warping

•

Balance

Firing frequency is a term used to describe the pulses created as the engine fires each cylinder. All engines have a firing frequency, but the object is to keep these disturbances from entering the passenger compartment. Initially, inspect for the following conditions:

Important: The frequency of these disturbances will depend on the number of cylinders. The engine order will always be equal to one-half the number of cylinders. This is because a four-stroke engine requires 2 complete revolutions of the crankshaft in order to fire all of the cylinders.

For example, a V8 engine will fire cylinders 1, 3, 5, and 7 on the first revolution. Cylinders 2, 4, 6, and 8 will fire on the second revolution. This results in 4 firing pulses per revolution of the crankshaft, or fourth order.

The following are symptoms of engine firing frequency:

The key to correcting these types of complaints is to isolate the vibration from the passenger compartment or the body.

1. Inspect for collapsed engine mounts.

Replace the mounts as needed. Refer to one of the following procedures in Engine Mechanical:

•	Engine Mount Replacement Front Mount

•	Engine Mount Replacement Front

•	Engine Mount Replacement

•	Engine Mount Replacement Right

•	Engine Mount Replacement Front

2. Inspect for a collapsed or broken transmission mount.

Replace as needed. Refer to 1999 4L60-E Transmission Replacement or 1999 4L80-E (G-Van) Transmission Replacement in Automatic Transmission.

3. Inspect the exhaust hangers.

Exhaust systems can expand 1-2 in when hot.

3.1.	Remove the hangers one at a time.

3.2.	Adjust or replace as needed.

4. Lubricate the manifold flexible coupling (if applicable).
5. Inspect the A/C and P/S lines.

5.1.	Isolate the hoses from the body.

5.2.	Install the retainers.

6. Inspect the drive belt for whipping.
7. Inspect the accessory unit fasteners for damage or looseness.
8. Inspect the body panels for missing or loose welds.
9. Remove any aftermarket equipment that may complete a transfer path into the passenger compartment.
10. Inspect the exhaust system and powertrain mounts. If necessary, re-bed the engine and align the exhaust system as follows:

10.1.

Loosen the mounts and hangers.

10.2.

Settle the powertrain by shifting from DRIVE to REVERSE.

10.3.

Tighten all fasteners with the powertrain in the relaxed position.

Some residual vibrations may be normal. Compare the vibrations with a similar vehicle in order to get a feel for what is commercially acceptable, preferably with the customer present. Refer to bulletins for updates on the use of mass damper weights for specific applications.

Just like firing frequency, some engine disturbances are the result of normal operation. Some engines can have additional inherent vibration due to the following elements:

Before attempting to repair a disturbance, compare the disturbance to another known good vehicle. Although some vibrations could be normal, the vibrations may not be pleasant to the customer.

The key to resolve an inherent engine disturbance is to isolate the vibration from the passenger compartment. Follow the same procedures as outlined for the engine firing frequency-related vibrations. Refer to Engine Firing Frequency-Related Vibration Causing Components.

Engine-driven accessories that exhibit vibration pose some special challenges. For example, the drive belts can no longer be removed one at a time in order to isolate the condition. If removing the belt eliminates the vibration, reinstall the belt and operate each accessory one at a time in order to see which has the most effect on the vibration. However, one component may affect another because the drive belt drives all of the accessories.

Inspect the accessory load to ensure that the load is not exciting the engine firing frequency.

Diagnosis

1. Compare the vibrations to a known good vehicle in order to verify that they are abnormal.
2. Remove the drive belt.
3. If the vibration stops, perform the following steps:

3.1.	Install the drive belt.

3.2.	Operate each accessory one at a time in order to see which has the most effect on the vibration.

4. Inspect for accessory load affecting the engine firing frequency.
5. Inspect the pulleys for misalignment or bends. Repair or replace as needed.
6. Inspect the A/C system for overcharging. Evacuate and recharge as needed. Refer toRefrigerant Recovery and Recharging in HVAC A/C Manual.
7. Inspect the engine oil level. Add or drain the oil as needed.

1. Raise and suitably support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
2. Follow the necessary safety precautions.
3. Attempt to duplicate the vibration.
4. While the vibration is present, find the areas of the vehicle that are excited by the vibration.
5. Inspect for witness marks due to a rubbing component.
6. Isolate the component and re-evaluate the vibration.
7. Inspect for proper torque on the excited components. Tighten as needed.
8. If the vibration still exists, refer to Engine Vibration Diagnosis with the EVA.

1. Place the EVA vibration sensor on the seat track rail with the UP label facing upward.
2. Plug the EVA into a 12 volt power supply.
3. Prepare the EVA for data recording:

3.1.	Press RECORD.

3.2.	Select a snapshot tag number.

3.3.	Press ENTER in order to begin recording.

4. Slowly accelerate until the vibration occurs.
5. Remember the vehicle and the engine rpm where the disturbance occurs.
6. Press ENTER on the EVA in order to record the vibration data.
7. Perform the road test diagnoses in order to determine engine-speed or vehicle-speed sensitivity. Refer to Types of Road Test Procedures .