Engine Noise Diagnosis. Pontiac Sun Runner - 2WD (Canada) 1998

For 1990-2009 cars

There are four steps to diagnosing engine noise. The technician must determine the following conditions:

• | The type of noise.

• | Under what condition the noise exists.

• | At what rate, and at what location in the engine the noise occurs.

• | Compare sounds in other engines to make sure the condition is not normal.

Remember, engine noises are generally synchronized to either the engine speed (indicating the noise is caused by the crankshaft, connecting rods or pistons) or one-half engine speed (indicating the cause is in the valve train).

### Main Bearing Noise

Damaged or worn main bearing noise is revealed by dull thuds or knocks which happen on every engine revolution. Bearing noise is loudest when the engine is under heavy load.

Excessive crankshaft end ply is indicated by an intermittent rap or knock that is sharper than the sound of a worn main bearing. Causes of a main bearing noise include the following:

• | Low oil pump pressure

• | Thin, diluted or dirty oil and/or filter

• | Excessive main bearing clearance

• | Excessive crankshaft end play

• | Out-of-round crankshaft journals

• | Excessive belt tension

• | Loose crankshaft pulley

• | Loose flywheel or torque converter

• | Loose main bearing cap

### Connecting Rod Bearing Noise

A damaged or worn connecting rod bearing will produce a knock under all speeds. During the early stages of wear, connecting rod noise may be confused with a 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 the following:

• | Excessive bearing clearance

• | Worn crankshaft connecting rod journal

• | Thin, diluted or dirty oil and/or filter

• | Low oil pressure

• | Crankshaft connecting rod journals out-of-round

• | Misaligned connecting rod

• | Connecting rod bolts not properly torqued

• | Wrong bearing inserts or misaligned bearing half

### Timing Belt and Sprocket Noise

Engines designed with timing belts and sprockets can produce different 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 belt and sprocket noises include the following:

• | Worn timing belt

• | Misaligned sprocket

• | Damaged sprocket

• | Sprocket loose on shaft

• | Too much end play in the camshaft or crankshaft

### Piston Noise

Piston pin, piston and connecting rod noises are hard to separate. A loose pin 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 not load on the engine.

Excessive piston-to-cylinder bore clearance will cause a 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-to-cylinder bore clearance is greater and piston slap will be louder. Causes of piston noises include the following:

• | Worn or loose piston pin or bushing

• | Improper fit of pin

• | Excessive piston-to-cylinder bore clearance (piston slap)

• | Lack of lubrication

• | Carbon deposits on top of the piston striking cylinder head

• | Worn or broken piston ring land

• | Broken or cracked piston

• | Misaligned connecting rods

• | Worn or damaged rings

• | Worn or out-of-round cylinder walls

• | Excessive ring land clearance

• | Insufficient ring-end gap clearance

• | Piston 180 degrees out of position

• | 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. Also, run the engine at various speeds and listen for an engine noise with the hood closed while sitting in the driver's seat.

The causes for valve noises include the following:

• | 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

• | Worn valve guides

• | Worn rocker arms

• | Broken rocker arm stud

• | Loose or worn rocker arm attachments

### Flywheel Noise (Automatic Transmission)

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.

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 32 km/h (20 mph) and shut off the engine. If a thud is heard, the flywheel may be loose or damaged. This type of thud is loudest upon deceleration.

A loose torque converter-to-flywheel or flywheel-to-crankshaft bolts will sound similar to bearing knock. These conditions produce 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. Check the torque converter-to-flywheel and flywheel to crankshaft bolts before attempting to investigate any bearing related knock.