Symptom
| Cause
|
There are four steps to diagnosing engine noise. You must determine
the following:
| | For example, is the noise a light rattle/tapping or a low rumble/knocking? |
| • | The exact operating condition under which the noise exists |
| | Note factors such as the ambient temperature, the amount of engine warm-up
time, the engine temperature, engine RPMs, and other specifics. |
| • | At what rate the noise occurs, and at what location in the engine |
| | Remember, engine noises are generally synchronized 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,
valve lifters, and timing chain). Try to determine the rate at which the noise
is occurring. |
| • | Compare the engine sounds to other engines and make sure you are
not trying to correct a normal condition. |
|
Noise on Start-Up but Only Lasts a Few Seconds
|
| • | Improper oil viscosity. Install the recommended oil viscosity
for the expected temperatures. |
| • | Incorrect oil filter without the anti-drainback feature |
| • | Worn or dirty valve lifters |
| • | Excessive piston to bore clearance |
| • | Excessive piston pin to piston clearance |
| • | Excessive crankshaft bearing clearance |
|
Knocks Cold and Continues for 1 to 2 Minutes
|
| • | Loose or broken crankshaft balancer or accessory drive components |
| • | Excessive piston to bore clearance |
| | A cold piston knock which disappears in 1.5 minutes should be considered
acceptable. |
| • | A cold engine knock usually disappears when the specific cylinders
secondary ignition circuit is grounded out. |
|
Intermittent Noise on Idle, Disappearing When Engine Speed is Increased
|
| • | Improper oil viscosity. Install the recommended oil viscosity
for the expected temperatures. |
| • | Lower than specified oil pressure |
| | Install an oil pressure gauge and measure the engine oil pressure. |
| • | Dirty or worn valve lifter |
|
Valve Train Noise (Rattle/Tapping)
| The following conditions may cause valve train noise:
| • | Lower than specified oil pressure |
| • | Worn or faulty oil pump |
| • | Loose oil pump-to-engine block bolts |
| • | Loose valve rocker arm attachments |
| • | Worn valve rocker arm bearings |
| • | Worn valve rocker arm and/or pushrod |
| • | Worn, dirty, or faulty valve lifters |
| • | Worn camshaft lifter lobes |
| • | Worn valve guides or valve stems |
| • | Bent, broken, or damaged timing chain sprocket teeth |
|
Knocks at Idle Hot (Rumble/Knocking)
| The following conditions may cause a knocking noise:
| • | Malfunctioning accessory drive system components |
| • | Loose or broken crankshaft balancer |
| • | Loose prop shaft hub or clamp (Corvette only) |
| • | Detonation or spark knock |
| | Check for proper operation of the cooling, knock, and ignition control
components. |
| | Refer to diagnostic information in Engine Controls. |
| • | Excessive connecting rod bearing clearance |
| • | Excessive piston pin to bore clearance |
| • | Excessive crankshaft main or thrust bearing clearance |
| • | Loose torque converter bolts |
| • | Cracked or damaged flywheel |
| • | Exhaust leak at the manifold |
|
Exhaust System Noise and/or Leakage
| Exhaust system noise and/or leakage may be caused by the
following conditions:
| • | Improperly installed or misaligned exhaust system components |
| • | A cracked or broken exhaust manifold |
| • | Damaged or worn gaskets and/or seals |
| • | Burned or rusted out exhaust system components |
| • | A broken or loose exhaust clamp and/or bracket |
|
