Knock Knock. Who’s there? It’s your engine, letting you know there’s a problem somewhere under the hood. If the smooth rumble you’re used to coming from your engine is replaced with a repetitive tapping or pinging sound that becomes louder and faster as you accelerate, that’s a classic sign of engine knock.
But “engine knock” can describe a variety of different noises that can each be caused by all sorts of individual problems.
Figuring out the source of the engine knock noise can be tricky! To help you get started, we’ve listed six of the most common causes of engine knock below.
What Is Engine Knock?
Engine knock often occurs when the air and fuel mixture in the engine cylinders is not properly ignited in a controlled manner, or when metal components are hitting together that shouldn’t be.
When an engine functions properly, fuel (usually gasoline) is mixed with air and then compressed. A spark plug ignites the compressed mix, which then combusts. The explosion pushes the engine pistons down, which drives the engine’s power. This process is repeated over and over to power the vehicle.
When you hear the engine knocking on acceleration or continuously while driving, something may be causing the combustion to occur at the wrong time. Keep reading to learn why this could happen.
What Does Engine Knock Sound Like?
Engine knock is not always as evident as a loud, thunking, knocking sound when driving down the road. In fact, it can be pretty subtle, and if you regularly cruise with your music at full blast, you may not even notice it!
Engine knock can be a metallic pinging, a repetitive tapping noise, or metal plinking against metal. Depending on the cause and severity of it, it can vary in loudness, frequency, and exactly when it occurs. The most obvious times are usually when you’re accelerating.
6 reasons why your car engine is knocking
If you’ve experienced your car making a knocking sound, then a problem is likely occurring upon combustion within the engine. There are several possible causes for this:
#1. Worn cambelt.
Also known as a timing belt, the cambelt synchronizes the rotation of various other components to ensure the engine valves open and close at the exact right moment. Eventually, a cambelt can become worn or frayed, causing a knocking sound inside your car.
Cambelt replacement requirements differ based on the car make and model. Take a look at the owner’s manual and if you’re in or approaching the recommended range, your cambelt could be the culprit.
#2. Low oil pressure.
Engine oil is a vital lubricant that keeps your motor running smoothly. Without it the engine will seize and the internal components of your engine can ‘knock’ against each other, which can lead to serious damage in the long term.
If the engine oil level is too low, the oil pump picks up air instead of oil, and air does not provide the lubrication needed to protect the moving parts of an engine. Knocking happens as a result of the bearings getting starved of oil, and they begin to melt from the heat of friction.
If your oil pressure is low, you may also have a leak. Try checking your oil and topping up if necessary. If you find it running low again quickly, this indicates a leak in the system, and you should contact a mechanic to organize a repair.
#3. Valve lifter failure.
A valve lifter is another essential engine component that transfers motion to open and close the valves. If your oil hasn’t been changed in a while, dirt can accumulate in the lifters, leading to a persistent knocking noise every time you start the engine. Lifters can also become clogged when you use the incorrect grade of oil for your engine.
Changing your oil and running a cleaning agent through the engine could be the simple solution to your engine knocking sound. Your vehicle’s handbook will give guidance as to how often oil will need changing for your particular vehicle and also the type of oil to use.
#4. Old spark plugs.
Spark plugs generate the electricity needed to ignite combustion in your engine. Without them, your car wouldn’t start. While spark plugs typically last thousands of miles, they do eventually need replacing. Your vehicle manual will specify how often you need to replace yours.
If your engine knocking sound is accompanied by other issues such as misfiring, stalling, struggling to start, and a ‘check engine’ light, it’s time to book in with a mechanic to have your spark plugs replaced.
#5. Bad fuel pump.
Used to transfer petrol or diesel from the fuel tank to the engine, a malfunctioning fuel pump is a common cause of engine knocking. When everything is working correctly, fuel is transferred into the combustion chamber where it is mixed with air – the oxygen needed for ignition.
When there is too much air inside the chamber, you may hear a knocking sound. This is known as running rich and is likely caused by a broken fuel pump.
In this case, you’ll need to consult with a trained mechanic for a fuel pump replacement.
#6. Worn engine bearings.
Engine bearings are there to reduce friction while supporting moving machinery. They facilitate the smooth movement of the pistons as they travel up and down in the engine, turning the crankshaft which transfers power to the wheels.
Over time, they can become worn out or come out of position, creating a rattling noise as the pistons clunk against the crankshaft.
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.
#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.
#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.