Complete the following steps in order to diagnose engine
noise:
- Determine the type of noise.
- Determine the condition under which the noise exists.
- Determine the rate and the location of the noise.
- Compare sounds in other engines in order to ensure that the condition
is not normal.
Engine noises are usually synchronized in one of the two following ways:
| • | The same as the engine speed |
| | This indicates that the noise is caused by one of the following components: |
| • | One-half of the engine speed |
| | This indicates that the valve train causes the noise. |
Main Bearing Noise
Listen for any of the following noises:
| • | Dull thuds or knocks that occur on every engine revolution |
| | This noise indicates that the bearings are damaged or worn. |
| | Any noise that is caused by the bearings is loudest when the engine
is under heavy load. |
| • | An intermittent rap or a knock that is sharper than the noise
that is caused by a worn main bearing |
| | This noise indicates excessive crankshaft end play. |
Any of the following conditions may cause main bearing noise:
| • | Excessive main bearing clearance |
| • | Excessive crankshaft end play |
| • | Out-of-round crankshaft journals |
| • | A loose crankshaft pulley |
| • | A loose torque converter |
| • | A loose main bearing cap |
Connecting Rod Bearing Noise
A damaged or worn connecting rod bearing causes a knock. The knock occurs
at all vehicle speeds.
The noise caused by the connecting rod may sound similar to the noise
caused by a piston slap or by loose wrist pins during the early stages of
wear.
Connecting rod knock noise becomes louder as the engine speed increases.
The connecting rod knock noise is loudest during deceleration.
Any of the following conditions may cause noise in the connecting rod
bearings:
| • | Excessive bearing clearance |
| • | A worn crankshaft connecting the rod journal |
| • | Crankshaft connecting rod journals that are out-of-round |
| • | A misaligned connecting rod |
| • | Improperly torqued connecting rod bolts |
| • | Incorrect bearing inserts |
| • | A misaligned bearing half |
Piston Noise
The following noises are similar:
A loose pin may cause a sharp double knock when the following conditions
exist:
| • | The engine suddenly accelerates then decelerates. |
An improperly fitted piston pin will not load on the engine.
Excessive piston-to-cylinder bore clearance causes a piston slap noise.
The noise is similar to a metallic knock. The piston slaps the cylinder wall
during the piston stroke. A decrease in noise as the engine warms up
indicates piston slap. When the engine is cold, the piston-to-cylinder
bore clearance is greater and the piston slap is louder.
The following conditions may cause piston noise:
| • | A worn or loose piston pin |
| • | A worn or loose bushing |
| • | A pin with an improper fit |
| • | Excessive clearance between the piston and the cylinder bore (piston
slap) |
| • | Insufficient lubrication |
| • | Carbon deposits on top of the piston striking cylinder head |
| • | A worn or broken piston ring land |
| • | A broken or cracked piston |
| • | Misaligned connecting rods |
| • | Worn or out-of-round cylinder walls |
| • | Excessive ring land clearance |
| • | Insufficient ring-end gap clearance |
| • | A piston that is 180 degrees out of position |
| • | An incorrect piston cam grind |
Valve Mechanism or Valve Train Noise
Either of the following noises may indicate a valve train condition:
| • | A light tapping at one-half of the engine speed |
| • | A light tapping at a varying frequency |
These tapping noises increase with engine speed.
Complete the following actions before attempting to evaluate valve train
noises:
| • | Thoroughly warm up the engine. |
| | A warm engine will ensure that all of the engine components are at normal
state of expansion. |
| • | Run the engine at various speeds. |
| • | Close the hood. Sit in the driver seat and listen for any engine
noise. |
Any of the following conditions may cause valve noise:
| • | Broken or weak valve springs |
| • | Sticking or warped valves |
| • | Dirty, stuck, worn or faulty valve lifters |
| • | Damaged or improperly machined camshaft lobes |
| • | Insufficient or poor oil supply valve train (low oil pressure) |
| • | Excessive valve stem-to-guide clearance |
