Engine Firing Frequency

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:
- The engine and the exhaust system are mounted in a relaxed position.
- All of the mounts and hangers should be in good condition and correct for the application.
- No components, hoses, or lines are grounding to the frame or to the body.

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.

Engine Firing Frequency Symptoms
The following are symptoms of engine firing frequency:
- The vibration may be torque sensitive.
- The vibration is engine-speed related.
- The vibration is heard as a boom or a moan.
- The vibration is felt as a shake, roughness, or buzz (depending on the number of cylinders).
- The vibration excites the resonance of a system or a component, causing the system or the component to have a narrow rpm range.
The key to correcting these types of complaints is to isolate the vibration from the passenger compartment or the body.

Engine Firing Frequency-Related Vibration Causing Components
1. Inspect for collapsed engine mounts.
Replace as needed. Refer to the appropriate procedure:
- Engine Mount Replacement (Front Right). in Engine Mechanical-2.2L
- Engine Mount Replacement (Front Left). in Engine Mechanical-2.2L
2. Inspect for a collapsed or broken transmission mount.
Replace as needed.
3. Inspect the exhaust hangers.
Exhaust systems can expand 1-2 in when hot.
1. Remove the hangers one at a time.
2. Adjust or replace as needed.
4. Lubricate the manifold flexible coupling (if applicable).
5. Inspect the A/C and P/S lines.
1. Isolate the hoses from the body.
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:
1. Loosen the mounts and hangers.
2. Settle the powertrain by shifting from DRIVE to REVERSE.
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.