6 Things That Can Cause Engine Knock in Your Car

What Is an Engine Knock?

Knocking occurs when fuel burns unevenly in your engine’s cylinders. When cylinders have the correct balance of air and fuel, fuel will burn in small, regulated pockets instead of all at once. (Think sparklers, not fireworks.)

After each pocket burns, it creates a little shock, igniting the next pocket and continuing the cycle. Engine knocking happens when fuel burns unevenly and those shocks go off at the wrong time. The result? An annoying noise and potential damage to your engine’s cylinder walls and pistons.

In other words, knocking (also knock, detonation, spark knock, pinging, or pinking) in spark-ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.

The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston’s stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle.

The shock wave creates the characteristic metallic “pinging” sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

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Knocking should not be confused with pre-ignition—they are two separate events. However, pre-ignition can be followed by knocking.

Engine knocking

What Could Cause Engine Knocking?

1. Too Low Octane Fuel

First, we need to know what detonation knock is. When your engine is running smoothly, the air/fuel mixture burns up in a single, controlled detonation inside each cylinder.

Detonation knock is a knocking noise that you’ll hear when the air-fuel mixture in the cylinders is detonating in more than one place at a time. Parts of the air/fuel mixture can start to ignite too early. When these mini fireballs collide, they create a knocking noise.

If your car has a performance-tuned engine rated for high-octane fuels, you could experience engine knock if you put in fuel with too low of an octane rating. High octane fuels burn more uniformly and resist knock.

So, if you put regular unleaded in your car that demands premium fuel, this could be the source of your knocking sound. Be sure to check your owner’s manual to find out what octane rating is right for your car.

2. Bad Timing

On most modern cars, the timing of the engine — that is, at which point in the engine’s travel the spark plugs will fire — is controlled by the computer. But, if for any reason the spark isn’t firing exactly when it should, this can cause multiple detonations in the cylinder, leading to engine knock.

3. Lean Air/Fuel Mixture

Problems with the oxygen sensors, fuel injectors, fuel pump, or mass airflow sensor can create a lean air/fuel mixture in the engine.

A lean air/fuel mixture is one that doesn’t have enough fuel and too much air. Without enough fuel in each cylinder, the mixture won’t burn fast enough, allowing for multiple detonations — that’s engine knock.

4. Bad Knock Sensor

Luckily, engine knock isn’t a frequent problem with modern cars, because the air/fuel ratio, fuel injectors, and timing are all computer-controlled. There’s even a knock sensor that’s designed to detect engine knock and tell the Engine Control Unit, so it can correct the problem automatically.

This naturally means that a bad knock sensor can allow the engine to knock. If you’re driving around in a modern car with engine knock, checking the knock sensor is an important part of the diagnostic process.

5. Rod Knock: Worn Bearings

Another type of engine knock is rod knock. As the pistons travel up and down in the engine, they turn the crankshaft which ultimately sends power to the wheels. The rod bearings facilitate smooth piston movement, but they can become worn out or come out of position over time.

As the bearings wear out, the pistons will start to rattle against the crankshaft, creating a very similar knocking sound.

To fix this problem, you may need new bearings or other work done on the pistons or crankshaft — parts located deep in the engine, meaning this can be a time-intensive repair.

6. Bad Belt Tensioners/Pulleys

Another possible source of engine knocking isn’t actually coming from the engine itself at all. It could be coming from the accessory belt. As the engine runs, it turns a belt that’s connected to various pulleys throughout the engine bay.

This belt must be under the exact right amount of tension so that it turns smoothly and quietly. If the belt becomes stretched out, the tensioner isn’t working properly, or one of the pulleys becomes bent out of shape, you can hear rattling, clicking, and slapping noises that might be mistaken for engine knock.

Luckily, fixing a noisy accessory belt can be as simple as replacing the belt, adjusting the tension, or replacing a bad tensioner or accessory pulley.

How to Fix Engine Knocking?

If you recently heard engine knocking coming from underneath the hood of your car, you shouldn’t waste any time when it comes to spring into action. The sooner you can figure out how to fix engine knocking, the better!

Fortunately, there are some fairly simple steps you can take to prevent engine knocking from taking a toll on your car. Some of the ways in which you can fix engine knocking include:

  • Upgrading the fuel that you put into your car and going with something that has a much higher-octane rating
  • Putting additives into your car that are designed to clean carbon buildups
  • Taking your car in for a tune-up and asking a mechanic to provide you with new spark plugs and spark plug wires

Doing something as simple as changing your oil on a more regular basis might also work wonders for your car and prevent engine knocks. If your engine isn’t lubricated properly, it can throw off the timing of many of your engine’s parts and cause complications.

Who Should I Trust to Fix Engine Knocking?

There are some ways in which you try to fix engine knocking on your own without the help of a mechanic. As we talked about a few seconds ago, changing the fuel that you put into your car is an easy step that you can take to eliminate engine knocking.

But if you try your best to stop engine knocking and can’t seem to figure out how to fix engine knocking, it’ll be time to bring in a professional to take a look at your car. They’ll be able to inspect your engine and find out what the problem is.

They might be able to make a small adjustment that goes a long way as far as fixing your engine knocking is concerned, or they might recommend more advanced repairs that will require their expertise.

How Much Does It Cost to Fix Engine Knocking?

Engine knocking tends to send shivers up and down people’s spines. And can you blame them? It’s just about never a good sign when you hear your engine making strange sounds! It’s usually a sign that your engine is going to require expensive repairs that you might not be able to afford.

But that isn’t always the case. As we touched on, there are instances in which replacing the spark plugs in a car can fix engine knocking. That will only cost you a couple of hundred dollars in most cases, and it won’t require you to give up your car for very long.

But if you allow your engine knock to stick around for too long, it could develop into a much bigger repair job. You could find yourself with worn-out engine bearings on your hands before too long, which could be a repair job that’ll cost you a couple of thousand dollars, if not more.

Is Fixing Engine Knocking Always Worth It?

If you learn how to fix engine knocking as soon as you notice it and find that you only have a minor problem that is causing your engine knock, making repairs to your car shouldn’t be too expensive. You should go ahead and try to fix the problem so that you can continue to drive your car around without any further issues.

But if you find that your engine knocking is being caused by a bigger issue that’s going to cost a small fortune to fix, making repairs is probably going to be out of the question. This is going to be especially true if you have an older car that isn’t worth very much money.

You don’t want to sink a bunch of cash into a car with an engine that knocks since it could be a sign that other problems are going to start to creep up before long.