What is Engine Knocking and How Can I Fix It?

If there’s something everyone can agree on, it’s that cars can make weird noises. It could be a squeal, a clunk, or even a rumble and roar. If the sound is closer to a “knock,” you may be dealing with problems under the hood—problems that could cause engine damage if left unresolved. Read on to learn what causes engine knocking and why it’s truly no joking matter.

What Is an Engine Knocking?

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.

Engine knocking

Why Does Engine Knock Happen?

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. Faulty spark plugs.

Spark plugs deliver the spark that ignites the air/fuel mixture mentioned above. Because they get a good workout every time you start your car, spark plugs just naturally wear out.

Their longevity can vary, depending on the type of spark plug and your driving style, but manufacturers generally recommend having new spark plugs installed every 30,000 to 100,000 miles. (Consult your vehicle owner’s manual for your car’s specific requirements.)

Why this matters: In addition to engine knock, worn or faulty spark plugs can lead to a loss of engine power and fuel economy.

What you can do: Spark plugs are affordable and readily available, so replacing them isn’t typically time or cost intensive. Regular tune-ups can help restore your car’s power and fuel efficiency as well as help prevent a small problem from becoming a costly repair.

# 2. Low-octane fuel.

Gasoline comes with different octane ratings or numbers. The number is a measure of the fuel’s ability to resist pre-ignition.

You want the fuel to burn, but at precisely the right time, as we explained above under, “What is engine knock?” The higher the octane, the better for that controlled, progressive burn.

If your car’s engine was designed to handle high-octane fuel and you substitute lower-cost, regular-grade fuel, you could be causing pre-ignition and engine knock.

Why this matters: High-octane gas is recommended for some vehicles. Drivers often choose to save money by filling their tanks with less expensive regular fuel. That’s not the best way to go! Long-term use of the wrong fuel can lead to engine knock, as well as engine damage and — get ready — decreased fuel economy. So those economy-conscious drivers may not be saving any money at all over the long run.

What you can do: First, check your owner’s manual. What’s your recommended fuel type and are you using it? If needed, step up your octane level at your next fill up or use an octane booster to increase performance. If this doesn’t seem to help after a few fill-ups, your problem might stem from something else.

#3. Carbon deposits.

All fuel sold in the United States is formulated with detergents that work to keep the engine clean. Unfortunately, carbon deposits can still form on the cylinders. These deposits can impact the air/fuel mixture, leading to incomplete combustion which can cause (you guessed it!) engine knock.

Why this matters: Carbon build-up can interrupt the combustion process, which can damage the engine’s cylinders. In addition to engine knock, you may notice lower fuel economy and a decrease in engine performance. More serious is the increased threat of your engine overheating because of carbon build-up.

What you can do: Clean the cylinders (aka the combustion chambers). If you’re not big on DIY — not everyone has the time or expertise for a project like this — bring your car to mechanic. Let trained technicians perform a complete tune-up, removing the carbon deposits to help your engine perform at an optimal level.

#4. 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.

#5. 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.

How to Fix Engine Knocking?

  1. The first step in trying to fix engine knocking is to upgrade to gasoline with a higher octane rating. Higher-octane fuels can withstand more pressure before combustion. Switch to one that includes carbon-cleaning additives. This can prevent further buildup on the various parts in your vehicle’s engine that effect timing. In the short term, a bottle of carbon-cleaning additive can help scour the system and expedite the resolution of causes 1, 3 and 5.
  2. Give your engine a tune-up that includes new spark plugs and wires.
  3. Regularly change the oil in your vehicle and monitor for a low oil level. Worn-out oil and low oil levels can contribute to poorly lubricated timing-related parts near the top of the engine.

How Much Does It Cost to Fix Engine Knocking?

Engine repairs are complex and can cost a bomb. Expect to shell out around $3000 to $5000 for a rod knock repair, depending on all these factors. However, the cost can go beyond $10000 if an engine replacement needs to be done. Rod knock repair requires replacing engine components like gaskets, seals, connecting rod bearings, etc., which are complex and expensive. Additionally, the cost to repair a rod knock also depends on your local labor costs.

Furthermore, you’d also require parts such as pistons, piston rings, crankshaft, camshaft, etc., which add up the high cost. Thus, replacing the engine is always better than repairing it if it requires a lot of part replacements, as it can save you a lot of money.

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.