What Is an Engine Knocking?
Engine knock is a tapping, pinging sound that gets louder and more obnoxious as you accelerate. 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 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.
Related: What is Internal Combustion Engine?
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.
Related: What is Spark Plug?
The shock wave creates the characteristic metallic “pinging” sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
Knocking should not be confused with pre-ignition, they are two separate events. However, pre-ignition can be followed by knocking.
6 Causes Of Knocking Sound From The Engine
When the steady rumble you are accustomed to hearing from your engine is replaced by a repetitive knocking or pinging that gets louder and faster as you accelerate, this is a classic sign of engine knock.
But “engine knocking” can describe a variety of different sounds, each of which can be caused by all sorts of individual problems. It can be difficult to pinpoint the source of the engine knocking noise! To get you started, we’ve listed six of the most common causes of engine knocking below.
1. Low-Octane or Low-Quality Fuel
First, we need to know what detonation knock is. When your engine is running smoothly, the air-fuel mixture combusts in each cylinder in a single, controlled detonation.
Detonation knock is a knocking sound you hear when the air-fuel mixture in the cylinders explodes in more than one place at a time. Parts of the air-fuel mixture can ignite too early. When these mini fireballs collide, they make a knocking sound.
If your car has a performance-tuned engine designed to run on high-octane fuel, you may experience engine knocking if you fill in fuel with an under-octane rating. High octane fuels burn more evenly and are more anti-knock.
So, if you regularly fill up your car with unleaded fuel that requires premium fuel, this could be the source of your knocking noise. Be sure to consult 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 are the engine to 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 an 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.
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?
Engine knocking is a sound only a mechanic is happy to hear. Most of us will have difficulty determining if the noise is rod knock, valve lifter tap, rocker arm, or spark knock. However, there are some steps automobile owners can do to fix engine noise before it causes major damage.
1. Change Oil and Filter
The first step is an oil change. Oil lubricates all moving parts in the engine and helps dissipate heat and removes metal chips and other debris. The oil filter removes most of the particulate matter, but sludge accumulates and can be recycled into engine parts.
Old, dirty oil can cause parts to stick, and unsuitable oil can be thinned by engine heat, creating a sticky sheen on cylinder walls and damaging other parts. Replace the oil and filter with manufacturer-recommended oil and filter every 5,000 to 8,000 miles. The old standard was every 3,000 miles or every 3 or 6 months. Some push that to every 10,000 miles, but that can be a stretch.
2. Use High Octane Fuel
Fuel type can affect engine wear and performance. While low octane fuel can be easy on the wallet, engine performance and efficiency can suffer. It could also be more expensive to repair an engine failure.
If you experience engine knock, make sure you are using the fuel type recommended by the manufacturer. You could also add an octane booster to improve the octane and hopefully kill the knock.
3. Add Fuel Detergent
Most automotive fuel contains some detergent to prevent carbon from building up on the cylinder walls and spark plug electrodes. Carbon buildup can cause hotspots and premature fuel-air detonation and knocking noise. Supplemental fuel cleaner can help remove carbon and eliminate engine knock.
4. Clean the Combustion Chamber
The accumulation of carbon deposits in the combustion chamber decreases the volume and increases compression. The accumulation disrupts the combustion process, reduces firing efficiency, and reduces the power cycle.
There are several combustion chamber cleaning products and procedures that you can perform. Watch some videos online and read and follow the product instructions. Alternatively, you can have this done by a mechanic you trust.
Related: What is Combustion Chamber?
5. Check or Replace the Spark Plugs
Spark plugs are designed for different temperatures and the gap between the electrodes must be just right. An improper spark plug can cause premature knocking and create spark knock due to its heat tolerance.
If the spark plug gap is too narrow or too far, there will be no spark at all. Carbon deposits can coat the electrodes and narrow the gap. Check and clean the spark plugs and ensure the vehicle is using the recommended spark plugs and that the gap is set correctly.
6. Reduce Intake Charge Density/Temperature
Cool air is denser and expands to a larger volume when heated, providing more oxygen for the fuel-air combustion mixture. A lower air intake temperature allows for a higher density engine intake charge, which improves combustion while reducing burn duration. Make sure the air intake is clear of dirt and away from hot engine areas such as the exhaust manifold.
Related: What is Exhaust Manifold?
7. Increase Engine Speed
Short, slow trips around town do not allow the engine to heat up and can lead to carbon deposits. Low RPM knocking can be caused by low octane fuel or stale fuel left in the engine for a long time. Make sure you are using the recommended grade of fuel and drive on the freeway for about an hour to clean the pipes.
8. Replace Knock Sensor
If you hear engine knocking, this could be due to a faulty knock sensor. The sensor isn’t in an easily accessible location, and most people don’t have the equipment to verify that it’s working properly. Take your car to a workshop you trust and have it checked and replaced if necessary.
9. Make Mixture Richer or Leaner
A richer mixture contains more fuel than the air-fuel mixture, while a lean mixture contains less fuel. The perfect mix burns all the fuel in the chamber leaving no oxygen behind. A rich mixture reduces knocking by lowering compression temperature and reducing the incidence of premature detonation.
The air gets thinner at higher altitudes. Due to the thinner air, a larger volume is required to burn a similar amount of fuel in the air-fuel mixture. A leaner mixture provides more complete combustion in the combustion chamber and less chance of detonation knock.
Who Should I Trust to Fix Engine Knocking?
There are some ways in which you try to fix an 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.
Engine knock is a tapping, pinging sound that gets louder and more obnoxious as you accelerate. 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.
Here, How to Fix Engine Knock:
1. Change Oil and Filter.
2. Use High Octane Fuel.
3. Add Fuel Detergent.
4. Clean the Combustion Chamber.
5. Check or Replace the Spark Plugs.
6. Reduce Intake Charge Density/Temperature.
7. Increase Engine Speed.
8. Replace Knock Sensor.
6 Things That Can Cause Engine Knock in Your Car:
1. Bad Belt Tensioners/Pulleys.
2. Rod Knock: Worn Bearings.
3. Detonation Knock: Bad Knock Sensor.
4. Detonation Knock: Lean Air/Fuel Mixture.
5. Detonation Knock: Bad Timing.
6. Detonation Knock: Too Low Octane.
Many things can cause engine knocking or banging. Common causes are: carbon buildup in the combustion chambers or on cylinder walls, lifters or connecting rods, combustion timing out of sync, overheated engine, back pressure from the exhaust, vacuum leaks, fuel octane or quality, spark plugs, or knock sensors failing.
Faulty Spark Plugs
Spark plugs are necessary for the burning air/fuel mixture to generate enough power for the engine to operate. Spark plugs age and becomes damaged over time, resulting in reduced spark. The sparks may be delayed at times, causing the engine to produce a knocking noise when accelerating.