What do you hear when you listen to your engine? The primal roar of high performance? Or is it more like a cry for help?
OK, we never want to think of our engines doing anything but chewing up asphalt and waking up the neighbors—let alone crying! But even the manliest of mills can have problems, and the sounds they make can help us track down the troubles. Sometimes it’s nothing; sometimes it’s serious.
We’ve put together this quick guide to diagnosing common engine noises to help you avoid potential damage. In the event of strange or unusual engine sounds, remain calm, grab an automotive stethoscope, and be on the lookout for:
Valve and tappet noise usually begins as a clicking sound, or chatter, at half engine speed and may then disappear at high speeds. The cause is often excessive valve clearance or a defective hydraulic valve lifter.
To check your clearances, you can insert a thickness gauge between the valve stem and the rocker arm or lifter. If this reduces the noise, the cause is excessive clearance, and you’ll need to make the proper adjustments. If it does not reduce the noise, the sound may be caused by worn lifter faces or rough cams. Other things to look for include lifters that are moving loosely in their bores and weak valve springs.
Detonation can cause serious damage to an engine. This condition shows up as a knocking or metallic “pinging” sound and is most commonly attributed to improper ignition timing, lean air/fuel ratio, or improper fuel octane level.
A somewhat common phenomenon in forced induction applications, detonation can be prevented by upping the octane level of your fuel, enriching the air/fuel mixture, reducing manifold pressure, or retarding the ignition timing. You can also consider an aftermarket water injection system for some applications.
Connecting Rod Noise
If you hear a light knocking or pounding sound, the noise can usually be traced back to your connecting rods. This sound is often most noticeable when the engine is at an even rpm–not accelerating or decelerating–and is often caused by a worn bearing or crankpin, misaligned connecting rod, or lack of oil.
You can single out the faulty connecting rod by performing a cylinder-balance test. This test basically shorts out the spark plugs one cylinder at a time with the engine running. Eventually, you’ll zero in on the ailing connecting rod because the noise will be reduced when its home cylinder is not delivering power.
Piston Pin Noise
Although similar to valvetrain noise, piston pin noise often has a unique, metallic-sounding double knock and is sometimes most noticeable during idle with the spark advanced. This noise is often caused by a worn or loose piston pin, worn bushing, or lack of oil.
As with connecting rod noise, you can find the offending components by performing the cylinder-balance test outlined above.
Piston Ring Noise
Piston ring noise is also similar to the valve and tappet noise above; however, it is most noticeable during acceleration. Most often, this noise is caused by low ring tension, broken or worn piston rings, or worn cylinder walls.
To troubleshoot each cylinder, remove the spark plugs and add a tablespoon of engine oil to each cylinder. Then, crank the engine for several revolutions to work the oil down past the rings. You can then install the spark plugs and start the engine. If the noise is reduced, the rings are probably the root of the problem.
A hollow, muffled, almost bell-like sound is usually piston slap. This condition is caused by a piston rocking back and forth within its cylinder. Continuous piston slap means the engine needs service; however, if you only notice this sound when the engine is cold, it is likely not serious.
A continuous piston slap sound is usually caused by worn pistons, excessive piston-to-wall clearance, misaligned connecting rods, worn cylinder walls, or inadequate oil.
A heavy, yet dull metallic knock is typically crankshaft knock. Loudest when the engine is under load or acceleration, crankshaft knock can be diagnosed by paying close attention to the specific type of knock:
- A regular, rumble-like knock is often from worn main bearings.
- A more distinct knock is routinely attributed to worn rod bearings.
- A sharp, irregular knock can be from a worn crankshaft thrust bearing.
This video may help too: Understanding the Difference Between “Knock” & “Rod Knock”