Why is my engine knocking? Three possible reasonsPosted by John Baker, 31st July 2017

There are a few different reasons why you might be dealing with a noisy engine – it could make a knocking, ticking or pinging noise, we’ve heard them all! Let’s break each down and talk about what might be happening.

Types of engine sounds - knock, tick or ping?

One driver’s engine knocking is another driver’s engine ticking. Or engine pinging. Some people even compare their engine sounds to marbles rolling around inside a coffee can.

The spontaneous ignition of the air/fuel inside the cylinders is a common source of abnormal engine sounds. While the description of the engine sound may differ, the causes are often the same: low-speed, high-torque conditions when you are accelerating.

Engine knocking typically occurs during low-speed, high-torque conditions, like when you are accelerating.

Say the clock has struck 5:00 pm and you make a beeline for your car to go home. When you mash the accelerator out of the parking lot, that’s when you hear your engine knocking. Or pinging. When you take your foot off the gas, it goes away.

This is likely due to either engine pre-ignition or detonation. They’re actually the same phenomenon, but they occur at different times.

In an engine that runs properly, spark-triggered ignition typically occurs a few degrees before the piston reaches top dead centre (TDC). This careful timing ensures the downward force of the exploding fuel/air mixture works in tandem with downward piston momentum, resulting in optimum efficiency and power.

Engine knocking - just bad timing?

Engine pre-ignition (and its cousin, low-speed pre-ignition [LSPI]) are abnormal combustion events that throw off this precise balance. Under certain conditions, the fuel/air can spontaneously ignite too early in the combustion cycle. Sometimes low-octane fuel is to blame, sometimes it is caused by deposits on the piston crown.

Car fuel with an octane rating too low for your engine can sporadically ignite prior to the piston reaching TDC. Or, chunks of carbon can heat up and create a hot spot that effectively ignites the fuel/air before the plug fires. Then, when the plug does fire a fraction of a second later, the two flame fronts collide. In certain conditions, they can clash with the upward-moving piston. The resulting shock wave rattles the piston inside the cylinder, creating the engine sounds – engine knocking, pinging or can-of-marbles, you call it!

Engine detonation has the same effect, except it occurs after the plug fires.

Computers in modern vehicles can detect engine knocking and compensate by adjusting engine timing. Though it saves your engine from destroying itself, performance and fuel economy can suffer.

Is your engine ticking like a time bomb?

If your engine is ticking like a time bomb, especially in the morning when it’s cold, your car likely has a valve-train issue.

Your engine uses intake valves to usher clean air into the cylinders and exhaust valves to kick the spent combustion gases out. The valves open and close thousands of time per minute in a choreographed whirlwind of activity.

A finely balanced system of parts – rocker arms, valve stems, cam lobes, lifters – control their movements. The clearances between these parts, known as lash, can become loose (or sloppy, in automobile jargon). When that happens, all those moving parts clattering against each other can make it sound like the engine ticking.

It’s especially noticeable in the morning, before the engine oil has had a chance to circulate through the upper end of the engine.

Many engines use hydraulic lifters, which use an oil-pressure-assisted plunger and spring to compensate for lash, helping to ensure that the system runs smoothly and to minimise engine noise.

Proper engine oil pressure plays a big role in valve-train operation and noise. Low oil pressure can reduce the effectiveness of hydraulic lifters, increasing lash. This is most likely to occur with a low quality conventional engine oil that thins at high temperatures, preventing the engine from developing good oil pressure.

If the rods are knocking…

Rod knock is yet another possible explanation for your engine noise.

Your engine is built with a designed clearance between the crankshaft journals and the connecting rods. In a properly running engine, and by using a good quality oil, the engine oil fills those clearances and prevents metal-to-metal contact.

But, let’s assume you’ve been using a poor quality conventional engine oil. At high temperatures, this poor quality oil thins and the fluid film weakens. The pressure between the crank journals and connecting rods squeezes the engine oil from the clearances. Now, metal is riding on metal, wearing the surfaces and widening the clearances. Eventually the clearances widen so much that you begin to hear the metal surfaces clattering against each other. Eventually, they’ll weld together and destroy the engine.

Quieting a noisy engine

This all sounds dire. But you can sometimes address pre-ignition by using a higher octane car fuel or by cleaning deposits from your engine.

Using a higher quality engine oil that flows better in cold weather and maintains its viscosity when hot can sometimes quiet a valve-train tick.

Rod knock is the worst of the three. Once the clearances between the crank journals and connecting rods have widened due to wear, it’s just a matter of time before catastrophic engine damage.

In any case, visit your mechanic and take care of the problem before it gets worse.

The moral of the story is simple: Pay a little more now to maintain your vehicle rather than spend a lot later to fix it. Use a high quality engine oil that stands up to extreme heat and maintains correct oil pressure.

Doing so can help keep your vehicle running properly and your engine quiet for years.

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