If only diagnosing a noise from the engine area was as simple as quickly listening to the vehicle and then ordering a new part. Unfortunately, there is often more to it than that. In our previous post we discussed understanding engine noises, and now we have put together this quick guide to diagnosing common car 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:
Valvetrain Noise
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 will 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, Pre-Ignition Noise
Detonation can cause serious damage to an engine. This condition shows up as a knocking or metallic “pinging” sound and is most 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 like 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 like 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.
Piston Slap
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.
Crankshaft Knock
A heavy, yet dull metallic knock is typically a 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.
Crankshaft Bearing Noise
- Crankshaft bearing noise is also caused by low oil pressure, which damages the bearing surfaces and could eventually damage the crankshaft itself. This type of noise is usually described as a rumbling or thumping sound deep in the engine when accelerating. If this sound is heard, it is extremely important that the engine not be run again until the oil pan is removed, and the crankshaft bearings are inspected. In many cases, the engine can be saved if the crankshaft is not damaged.
- A mechanic will fix the problem by replacing the bearings and solving the oil pressure problem. Bearing shells are what the crankshaft rotates within. If you continue to run the engine in this condition, you will most certainly cause a major engine failure. It is also a high possibility that when you hear the noise, it may be too late to save it without removing the engine. This can be an expensive repair in the four-figure cost range.
Timing Chain Noise
- Many of the newer engines have overhead camshafts with longer timing chains. A timing chain connects the crankshaft to the camshaft to ensure the valves open at the proper time. The slack in these chains is usually kept tight by hydraulic tensioners.
- The chains ride against a nylon guide (a chain guide) which, in time, begins to wear. At the point where the chain guides are worn beyond the ability of the hydraulic tensioner to take up the slack, the timing chain begins to rattle. This noise is caused by the timing chains becoming so loose that they whip back and forth against the guides and possibly the timing cover.
- If the oil pressure is correct, the replacement of the hydraulic tensioners and chain guides would be required. A mechanic’s stethoscope is a great tool to pinpoint this noise. If the noise is loudest when touching the timing cover with the stethoscope, disassembly would be required to confirm and repair the problem. This is a semi-major job for most of these engines and would usually cost in the four-digit range.
Whining Noise
- A whining noise when an engine is running is usually an indication of a bearing that is on the verge of failure. This noise will increase as the engine RPMs increase.
- Remember, though, that there are multiple places in the engine where bearings are used. Thus, there are several items under the hood that can make this kind of whining sound. The water pump, air conditioner clutch bearing (which will only be heard when the compressor is not engaged), fan belt idler pulleys or belt tensioner, alternator, and the power steering pump – are all possible sources for whining sounds indicating future bearing failure.
- Whining from the power steering pump will get louder when turning the steering wheel from side to side, and the usual cause is low power steering fluid. A mechanic’s stethoscope is the best way to diagnose the other bearing noises. Failure to repair any one of the items making a whining sound can result in a vehicle breakdown.
- Not only that, but other engine components can be damaged when parts with bearings eventually come apart. So, it is a smart idea to get this kind of problem checked out. Spend a little money now to save a lot of money later.
Finally
In summary, bottom end noise typically originates from the lower part of the engine, involving components like the crankshaft and connecting rods, and tends to be deeper and more serious. Top end noise comes from the upper part of the engine, involving components like the valves and camshaft, and is generally lighter and may be less severe. Identifying the source of car engine noises is crucial for diagnosing and addressing engine problems effectively.
