There are four steps to diagnosing engine noise. You must
determine the following conditions:
| • | Determine the exact operating condition under which the noise
exists. |
| • | At what rate, and at what location in the engine. |
| • | Compare sounds in other engines to make sure you are not trying
to correct a normal condition. |
Identify the type of noise. For example, a light rattle or low rumble.
Remember, engine noises are generally synchronized to either engine
speed (caused by the crankshaft, connecting rods or pistons) or one-half engine
speed (valve train noise). Try to determine the rate at which the noise is
occurring.
Main Bearing Noise
Damaged or worn main bearing noise is revealed by dull thuds or knocks
which happen on every engine revolution, and this noise is loudest when the
engine is under a heavy load.
Excessive crankshaft end play is indicated by an intermittent rap or
knock sharper than a worn main bearing. Causes of main bearing noise include:
| • | Thin, diluted, or dirty oil and/or oil filter |
| • | Excessive main bearing clearance |
| • | Out-of-round crankshaft journals |
| • | Loose crankshaft pulley |
| • | Loose flywheel or torque converter |
| • | Loose main bearing bolts. |
Connecting Rod Bearing Noise
A damaged or worn connecting rod bearing will produce knock under all
speeds. During the early stages of wear, connecting rod noise may be confused
with piston slap or loose wrist pins. Connecting rod knock noise increases
in volume with engine speed and is at its loudest on deceleration. Causes
of connecting rod bearing noise include:
| • | Excessive bearing clearance |
| • | Worn crankshaft connecting rod journal |
| • | Thin, diluted, or dirty oil and/or oil filter |
| • | Crankshaft connecting rod journals out-of-round |
| • | Misaligned connecting rod |
| • | Connecting rod bolts not properly torqued |
| • | Wrong bearing inserts or misaligned bearing half. |
Timing Chain and Sprocket Noises
The most common noise is a high frequency, light-knocking sound. This
sound will generally be the same in intensity whether the engine is idling,
operating at high speeds, or under load. Causes of timing chain and sprocket
noise include:
| • | Worn timing chain and/or gears |
| • | Gear or sprocket loose on shaft |
| • | Too much end play in the camshaft or crankshaft |
| • | Tensioner not functioning properly |
| • | Loose or improperly seated camshaft timing gears are usually loudest
when warm. They are speed sensitive only, not load sensitive. |
| • | Cam gear noise will usually be evident at a warm idle in neutral
and sounds much like a loose timing chain noise. |
Piston Noises
Piston pin, piston, and connecting rod noises are hard to separate.
A loose piston pin, for example, causes a sharp double knock usually heard
when the engine is idling, or during sudden acceleration then deceleration
of the engine. A piston pin that has been improperly fitted will emit a light
ticking noise that is more noticeable with no load on the engine. Excessive
piston-to-cylinder bore clearance will cause piston slap noise. The noise
is similar to a metallic knock, as if the piston were "slapping"
the cylinder wall during its stroke. As with most engine noises, understanding
the cause of the noise will help you imagine what the noise sounds like.
An indication of piston slap is a decrease in noise as the engine warms up.
When the engine is cold, the piston bore clearance is greater and piston
slap will be louder. Causes of piston noises include:
| • | Worn or loose piston pin or bushing |
| • | Excessive piston-to-cylinder bore clearance |
| • | Carbon deposits on top of piston striking the cylinder head |
| • | Worn or broken piston ring land |
| • | Broken or cracked piston |
| • | Misaligned connecting rods |
| • | Worn or out-of-round cylinder walls |
| • | Excessive ring land clearance |
| • | Insufficient ring-end gap clearance |
| • | Piston 180 degrees out of position |
Valve Mechanism or Valve Train Noises
A light tapping at one-half engine speed, or any varying frequency,
can indicate a valve train problem. These tapping noises increase with engine
speed.
Before attempting to judge valve train noises, thoroughly warm up the
engine. By doing this you will bring all components to a normal state of expansion.
Also, run the engine at various speeds and listen for engine noise with the
hood closed while sitting in the driver's seat.
After a major engine repair or long periods of non-use, "Lifter
Clatter" at initial start-up is normal. This should go away after 10-15
minutes of operating the engine at 1500 RPM. Operating the engine in this
manner will purge all air out of the lifters.
Caution: Do not remove the camshaft covers to diagnose valve train noises because
exhaust manifold temperature will exceed the flash point of the oil causing
a fire with possible personal injury.
If the valve mechanism is abnormally noisy, use a stethoscope to determine
which valves or valve train parts are causing the noise.
The causes of valve mechanism noise include:
| • | Broken or weak valve springs |
| • | Sticking or warped valves |
| • | Dirty, stuck or worn valve lifters |
| • | Damaged or improperly machined camshaft lobes |
| • | Insufficient or poor oil supply to the valve train (low oil pressure) |
| • | Excessive valve stem-to-guide clearance |
Flywheel Noises
A loose or cracked flywheel will produce an irregular thud or click.
To test for a loose or cracked flywheel, operate the vehicle at approximately
20 mph (32 km/h) and shut off the engine. If a thud is heard, the flywheel
may be loose or damaged. This type of thud is loudest on deceleration.
Loose torque converter-to-flywheel or flywheel-to-crankshaft bolts will
sound similar to bearing knock. This condition produces several raps during
quick acceleration on a free running engine. Depending on the idle smoothness,
when the transaxle is in gear, the noise may or may not appear.
Notice: Be sure the converter-to-flywheel bolts are not too long. Converter
bolts that are too long may dimple the torque converter clutch apply surface
causing a shudder condition.
Check the torque converter-to-flywheel and flywheel-to-crankshaft bolts
before attempting to investigate any bearing related knock.
