Cause
| Correction
|
When diagnosing engine noise complaints use the following steps to isolate
the source of the engine noise:
- Determine the type of the noise.
This may include a light rattle, tapping or low rumble or knock.
- Verify the exact operating condition when the noise occurs.
This would include ambient engine temperature, engine warm-up time,
engine operating temperature, and engine RPM.
- Isolate the rate when the noise occurs.
Generally speaking, engine noises are timed to either engine speed (crankshaft,
flywheel, connecting rods, balancer, or pistons and related components) or
one-half engine speed (valve train noise such as rocker arms, valves,
and timing belt).
- Compare the engine sounds to a known good engine to prevent trying
to fix a noise that is normal.
|
Noise on Start-Up but Lasts a Few
Seconds
| The
following condition(s) may produce engine knocks on initial start-up, but
only last a few seconds:
| • | Improper oil viscosity. |
| • | Install proper oil viscosity for expected temperatures. Refer
to Maintenance and Lubrication |
|
Knocks Cold and Continues for 1
to 2 Minutes
|
| • | Excessive piston to bore clearance. Replace the piston. |
| • | Cold engine knock usually disappears when the specific cylinder
secondary ignition circuit is grounded out. Cold engine piston knock which
disappears in 1.5 minutes should be considered acceptable. |
|
Valve Train Noise (Rattle/Tapping)
| The following conditions may cause valve train noise:
| • | Loose or misadjusted valve rocker arms. |
| • | Worn valve rocker arm and/or rocker arm shaft. |
| • | Worn or damaged valve rotators. |
| • | Sticking valves. Inspect the valves. |
| • | Worn or faulty camshaft. Replace the camshaft. |
| • | Worn or faulty oil pump |
| • | Worn valve guides. Repair as necessary. |
| • | Oil overfull causing foaming of oil. |
|
Knocks at Idle
Hot (Rumble/Knocking)
|
| • | Inspect the drive belts for wear. Check the tension and/or replace
as necessary. |
| • | Inspect the A/C compressor or generator bearing. Replace or repair
as necessary. |
| • | Inspect the valve train. Replace the parts as necessary. |
| • | Inspect for improper oil viscosity. Install proper viscosity oil
for expected temperature. Refer to Maintenance and Lubrication for engine
oil specifications. |
| • | Inspect for excessive piston pin clearance. Replace the piston
and pin as necessary. |
| • | Inspect the connecting rod alignment. Check and replace the rods
as necessary. |
| • | Inspect for insufficient piston-to bore clearance. |
| • | Bore, Hone, and fit the new piston. |
| • | Inspect the crankshaft balancer for looseness. Torque and/or replace
the worn parts. |
| • | Ensure that the piston pin is not offset to the wrong side. Install
the correct piston. |
|
Light Knock Hot
|
| • | Exhaust leak at the manifold. Tighten the bolts and/or replace
the gasket. |
| • | Excessive rod bearing clearance. Replace bearings as necessary. |
|
Noise at High Vehicle Speeds, Quiet at Low Speeds
| Noise at high vehicle speeds may
be caused by the following conditions:
| • | High oil level. Oil levels above the FULL mark allows the crankshaft
counterweights to churn the oil into foam. Drain the oil to the proper level. |
| • | A low oil level. Oil below the ADD mark allows the oil pump to
pump air at high speeds. Add oil as necessary. |
| • | Oil pan bent up against the oil pump pickup screen. |
| • | Oil pump pickup screen bent or loose. |
|
Noise Regardless
of Engine Speed
|
| • | Engine flywheel contacting the splash shield. Reposition the splash
shield. |
| • | Loose or broken crankshaft balancer or drive pulleys. Tighten
or replace as necessary. |
| • | Incorrect valve adjustment leading to noise regardless of engine
speed. Check valve lash. |
| • | Excessive valve lash may cause engine noise. |
| • | Excessive valve lash may be caused by the following conditions: |
| - | A worn or damaged camshaft. Replace the camshaft. |
| - | A worn or bent rocker arm. Replace the rocker arm. |
| - | A loose or damaged rocker arm. |
| • | Check the lubrication system feed to the camshaft. |
|
Vibrating or Rattling from
Exhaust System
| Vibration or ratting from the exhaust system may be caused by loose
and/or misaligned exhaust components. Align, then tighten the connections.
Check for damaged hangers or mounting brackets and clamps.
|
Exhaust Leakage
and/or Noise
| Exhaust leakage and/or noise may be caused by the following conditions:
| • | Leakage at the exhaust component joints and couplings. Tighten
the clamps or couplings to the specified torque. |
| • | Improperly installed or misalignment of the exhaust system. Align,
then tighten the exhaust clamps. |
| • | A cracked or broken exhaust manifold. Replace the exhaust manifold. |
| • | A leak between the exhaust manifold or the cylinder head. Tighten
the exhaust manifold to the cylinder head nuts and bolts to specifications. |
| • | Damaged or worn exhaust seals. Replace as necessary. |
| • | A burned or rusted out exhaust pipe. Replace the exhaust pipe
as necessary. |
| • | A burned or blown out muffler. Replace the muffler assembly. |
| • | A broken or loose exhaust clamp and/or bracket. Replace as necessary. |
| • | Loose or damaged oxygen sensors. |
|
Heavy Knock with
Torque Applied
|
| • | Broken balancer or pulley hub. Replace parts as necessary. |
| • | Accessory belts too tight or nicked. Replace and/or tension to
specifications as necessary. |
| • | Excessive main bearing clearance. Replace as necessary. |
| • | Excessive rod bearing clearance. Replace as necessary. |
|
Main Bearing Noise
| Damaged or worn main bearing noise is revealed by dull thuds or knocks
that occur on every engine revolution. This noise is loudest when the engine
is under heavy load.
Excessive crankshaft end play is indicated by an intermittent rap or
a knock that is sharper than the sound of a worn main bearing
Any of the following conditions may cause main bearing noise:
| • | Diluted, thin or dirty oil |
| • | Diluted, thin or dirty oil filter |
| • | Excessive main bearing clearance |
| • | Excessive crankshaft end play |
| • | Out-of-round crankshaft journals |
| • | A loose crankshaft pulley |
| • | A loose main bearing cap |
|
Connecting Rod Bearing Noise
| A damaged
or worn connecting rod bearing will produce a knock at all vehicle speeds.
During the early stages of wear, connecting rod noise may sound similar
to the noise caused by loose wrist pins or piston slap.
Connecting rod knock noise becomes louder as the engine speed increases
and is loudest on deceleration.
Any of the following conditions may cause connecting rod bearing noise:
| • | Excessive bearing clearance |
| • | A worn or out-of-round crankshaft connecting rod journal |
| • | Diluted, thin or dirty oil |
| • | Diluted, thin or dirty oil filter |
| • | A misaligned connecting rod |
| • | Improperly torqued connecting rod bolts |
| • | Incorrect bearing inserts |
| • | A misaligned bearing half |
|
Timing Belt and Sprocket
Noise
| Different
noises may be caused by engines that are designed with timing belts and sprockets
from those designed with timing gears or timing chains.
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.
Any of the following conditions may cause noise in the timing belt and
the sprocket:
| • | A sprocket that is loose on the shaft |
| • | Excessive end play in the camshaft or the crankshaft |
|
Piston Noise
| Piston pin, piston and connecting rod noises are hard to separate. A loose
piston pin causes a sharp double knock usually heard when the engine
is idling, or during sudden acceleration then deceleration of the engine.
Excessive piston-to-cylinder bore clearance causes a piston slap noise.
The noise is similar to a metallic knock as if the piston were slapping the
cylinder wall during its 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 piston pin 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 the 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
| 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 evaluate valve train noises, thoroughly warm up
the engine. This will bring all engine components to a normal state of expansion.
Run the engine at various speeds and listen for engine noises with
the hood closed while sitting in the driver's seat. The causes of
valve train noise include the following:
| • | Worn or misadjusted valve rocker arms |
| • | Broken or weak valve springs |
| • | Sticking or warped valves |
| • | Damaged or improperly machined camshaft lobes |
| • | Insufficient or poor oil supply to the valve train (low oil pressure) |
| • | Excessive valve stem-to-guide clearance |
|
