Many of us rely on motorized transportation to get us to and from work, groceries, entertainment, and sports events, just about everywhere we want to go. So, if you hear engine knocking sounds, it’s time to listen and get it repaired, or you may be walking everywhere.
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.
Ignoring the knocking won’t make it go away, and could lead to a blown engine. In this article, we discuss the causes and cures for engine knocking. By the time you complete reading, you’ll better understand the causes, how to repair them, and whether it’s safe to drive your ride with engine knock.
Contents (Jump to Topic)
What Is an Engine Knock?
Engines have many moving parts that make the cacophony of noise we commonly hear as we listen to an automobile engine. If you start hearing unusual sounds, it’s time to sit up and listen. The soft tap that increases or decreases with the pressure on the gas pedal is possibly the valves or tappets sticking.
Harder knocking sounds often indicate the ignition timing is off, especially if you notice a drop in power. A rattling, when starting the car, followed by the harder knocking noise, helps identify what is throwing the ignition timing off. Rattling that seems to come from under the engine will require more time to repair.
Identifying or differentiating between crankshaft or detonation knocking or noise from piston rings is beyond the knowledge of most drivers. A good service center should be able to help.
Engines that knock or ping identify other problems. Knocking that gets louder and ends with a bang followed by metallic grinding or screeching mean costly engine repairs. Engine knock coupled with the check engine light activating is a good indication you need to do some work or take your vehicle to a repair shop.
Engine Knocking Causes
A simplified explanation of how an engine works helps with an understanding of what could be knocking under the hood. A four-stroke automobile engine is a complex dance of parts working together as each piston goes through four movements. When you turn the key or push the starter, stored energy in the battery provides the power to the starter, which turns the crankshaft. The zig-zag shaped crankshaft is at the bottom of the engine and exposed to the oil pan for lubrication.
With each rotation of the crankshaft, the main bearings, connecting rod bearings, connecting rods, pistons, end bushings, cylinder, and rings are lubricated with oil. The rotation is responsible for intake, compression, combustion, and exhaust – one combustion in four movements, or a 1:4 power stroke ratio. While the crankshaft orchestrates the movement of the pistons, it also turns the intake and exhaust camshaft, or shafts.
The camshaft controls when the intake or exhaust valves are open or closed. It either moves the valve stem directly or by using valve lifters connected to adjustable push rods and a rocker arm. Each valve stem goes through a coil spring, which closes or re-seats the valve when the cam or rocker arm isn’t forcing it open. There is an oil seal on each valve shaft to prevent oil from getting into the combustion chamber. All moving parts are lubricated with oil from the oil pan reservoir.
Some engines have one set of intake and exhaust valves per cylinder; others have two sets. The greater the number of moving parts, the higher the RPMs, the greater the potential for knocking noise and engine damage. Once the pistons are firing to keep the crankshaft turning, the starter backs off, and the alternator recharges the battery for the next time it’s needed.
1. Low-Octane or Low-Quality Fuel
Using gasoline with a lower octane value than recommended for the vehicle can cause the fuel-air mixture to detonate prematurely, generating detonation knock. The result is a pinging or engine knocking sound. Different fuel types are assigned numbers and names based on their octane rating.
The higher the octane rating, the more refined the fuel, and the greater the percentage of octane. It has a more controlled detonation, so it is less likely to prematurely ignite. Manufacturers call octane levels by names and numbers – Regular or Bronze is 87% octane, Extra or Silver is 89%, and Supreme, Premium or Gold is 91% to 93%. In areas of higher elevations, octane 85 is commonly available too.
2. Incorrect or Faulty Spark Plugs
Spark plugs generate the ‘spark’ in the combustion chamber, igniting the fuel-air mixture for the power stroke of the engine. Faulty or incorrect plugs or the wrong spacing or gap may cause engine knocking. Spark plugs have different heat tolerances and may cause premature detonation, also known as detonation knock. If the gap is too narrow, the spark is too weak to ignite the mixture, and too wide a gap may not spark at all.
The current pulse sent to the spark plug to ignite the fuel-air mixture is computer-controlled on modern vehicles. In older cars, it is activated by the distributor cap. Faulty or incorrect plugs can result in bad timing of ignition in the combustion chamber, resulting in knocking too.
3. Bad Knock Sensor
Modern vehicles rely on microcomputers and sensors to keep the engine running smoothly. The Engine Control Unit (ECU) oversees the fuel injectors, fuel-air mixture, and timing. If there is any knocking, the knock sensor relays the information to the ECU and automatically corrects the problem. However, if the knock sensor is faulty, you may hear knocking.
4. Lean Air/Fuel Mixture
Combustion occurs when a spark ignites the compressed air-fuel mixture. If there is too much oxygen, then the fuel-air mixture is lean. That means there isn’t enough fuel in the mix to burn quickly, resulting in multiple detonations and a knocking sound. If left unchecked, the pistons and cylinder walls will be damaged.
The common causes link to engine components that regulate or control the flow of both air and fuel. Make sure the oxygen and mass airflow sensors are functioning properly, and check the fuel injectors and pump are in working order.
5. Worn Bearings
For every hour a vehicle operates at 1500 RPM, it rotates approximately 90,000 times. So, commuting an hour each way for work five days a week is 900,000 rotations – touch the pedal to pass and jump that to 3000 to 5000RPM or more, and that weekly number multiplies. In the course of a year, an engine can easily complete 50-million rotations, which can cause bearings to wear.
The main bearings between the crankshaft and the engine block, and the connecting rod bearings between the connecting rod and the crankshaft, execute millions of rotations. Over time, bearings wear and can lead to a rattling or knocking sound deep in the engine. If left unchecked, it can damage the connecting rods.
6. Engine Timing Belt or Chain Issues
The timing chain or belt links the crankshaft and camshaft and synchronizes the movement of the pistons with the opening and closing of the intake and exhaust valves. If the timing is off, spark may not happen when it should, leading to multiple explosions resulting in engine knocking.
Older vehicles have a distributor cap, which is also linked to the timing to ensure the electrical impulse goes to the spark plug to ignite the air-fuel mixture at the correct time. Modern engine timing uses computer-controls instead to determine when the spark will happen during the piston compression cycle of the air-fuel mixture. Combustion timing being off could be due to a computer malfunction in modern vehicles, instead of a mechanical problem in older autos.
7. Cylinder Head Is Not Adequately Lubricated
The cylinder head may generate a knocking sound if it doesn’t receive adequate lubrication. This commonly occurs if the oil is old or there is a leak, resulting in insufficient oil in the reservoir pan. It can be caused by using generic oils that have a lower flashpoint, meaning they will vaporize on the upper cylinder walls due to the high temperatures.
Many manufacturers recommend synthetic oil to keep the cylinder head lubricated. Inadequate lubrication can also cause damage to the piston rings and shellac like finish to the cylinder walls. The result can be decreased compression, power, and efficiencies.
8. Low Oil Pressure
Low oil pressure is another cause of engine knock. Oil lubricates and dissipates heat from moving parts such as the camshaft, cam lobes, lifters, valve stems, and rocker arms. The lash, or distance between parts, is saturated in oil to lubricate and fill the lash or voids. If the gaps get too big due to wear or low oil pressure, the components clatter or knock.
The knocking sound may be more noticeable on cold starts before the oil can fully circulate into all gaps within the engine. Low oil pressure also affects modern engines that use hydraulic lifters to minimize lash and thus knocking. High engine temperatures can cause low quality or low-grade oils to thin and interfere with the development of proper oil pressure.
The use of manufacturers recommended engine oil should produce proper oil pressure unless there is a leak somewhere in the oil system. Check the spark plugs for white ash and look for oil on the ground under the engine or around the seals to see if there’s an oil leak. Also, if the exhaust is bluish-black, oil is likely being burned within the combustion chambers.
9. Valve Lifter Failure
Faulty valve lifters produce a rapid tapping sound, whether the engine is cold or hot. The valve lifters (aka hydraulic tappet or hydraulic rod adjuster), ensure the clearance of all valves in the engine stay at zero. They are located at the end of each rocker or push arm, opposite the valve stem they assist.
A faulty lifter may stick, causing a clicking, ticking, or tapping sound heard over the engine noise. They sound and duration often indicate how bad the problem is. Tapping for a brief moment at start-up is frequently caused by dirty oil. If the noise doesn’t stop, the lifter valve will cause damage to the engine.
10. Carbon Deposits
Carbon-based fuels like gasoline and diesel, even with carbon cleaning additives or detergents, can still produce carbon buildup. The carbon can accumulate on spark plugs, valves, pistons, and the combustion chamber, and walls of the cylinder. The buildup decreases the chamber volume, increases compression, and reduces the efficiency of the firing and power stroke.
The carbon deposits may also create hotspots that can ignite the air-fuel mixture ahead of ignition by the spark plug. The double firings are slightly out of sync in the chamber, generating a shockwave as the piston reaches the top of its stroke. The result is a knock or ping or a rattle that sounds like a can of marbles. If the hotspot detonation occurs after the spark plug ignites the mixture, the results are the same.
The knock sensor on modern vehicles will detect engine knocks and relay the information to the Engine Control Unit (ECU). The ECU will adjust the timing to match the additional firing by the carbon. The adjustment protects the engine from major damage. Unfortunately, efficiency and performance are reduced.
Other Possible Causes
Here are some other components that may cause a knocking sound under the hood.
High Compression Ratio
A high compression ratio can generate higher combustion chamber temperatures resulting in the fuel-air mixture being detonated before the spark plug can ignite it. The ensuing explosion is out of sync and produces a knocking noise.
An overheating engine is often the result of a cooling system problem, such as a coolant leak or faulty water pump. The hot engine can cause pre-ignition, which produces a knocking sound as the air-fuel mixture explodes prematurely due to the high temperatures.
Low Speed of Engine
Engine knocking at low engine speeds of 20 to 30mph is often spark knocking caused by low octane fuel or old fuel.
Pulleys and Tensioners
Pulleys can be damaged and produce clicking or knocking noises. If belts become worn or loose, they may slap, tap, or rattle. The tensioners are frequently spring-loaded and apply pressure on a section of a belt to keep it taut. A loose or broken tensioner can rattle or tap too.
How to Fix Engine Knock
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 remove metal filings and other debris. The oil filter removes much of the particles, but sludge does build up and can recirculate into engine parts.
Old, dirty oil can cause parts to stick, and improper grade oil can thin with engine heat creating a sticky sheen on cylinder walls and damage to 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 may be a stretch.
2. Use High Octane Fuel
Fuel type can adversely affect engine wear and performance. While low octane fuel may be easier on the pocketbook, the engine performance and efficiency may suffer. It could also be more costly repairing engine damage. If you’re experiencing engine knock, make sure you are using the manufacturer’s recommended fuel type. You could also add an octane booster to improve the octane rating and, hopefully, end the knocking.
3. Add Fuel Detergent
Most automotive fuels contain some detergent to prevent carbon from accumulating on the cylinder walls and spark plug electrodes. Carbon build-up can cause hotspots and premature fuel-air detonation and a knocking sound. Additional fuel detergent may assist in removing carbon and eliminating engine knock.
4. Clean the Combustion Chamber
The buildup of carbon deposits in the combustion chamber will reduce the volume and increase compression. The buildup interferes with the combustion process, lowers the firing efficiency, and decreases the power stroke. There are different products and procedures for cleaning the combustion chamber that you can do. Check out some videos online and read and follow the product directions. Alternatively, have your trusted mechanic do it for you.
5. Check or Replace the Spark Plugs
Spark plugs are rated for different temperatures, and the gap between the electrodes must be just right. An incorrect spark plug can cause detonation to occur prematurely due to its heat tolerance, generating spark knock. If the spark plug gap is too narrow or wide, it won’t spark at all. Carbon buildup can coat the electrodes and narrow the gap. Check and clean the spark plugs, and ensure the vehicle is using recommended plugs and that the gap is correctly set.
6. Reduce Intake Charge Density/Temperature
Cool air is denser and will expand to a greater volume when heated, providing more oxygen to the fuel-air combustion mixture. A lower air intake temperature allows for a higher density engine intake charge, improving combustion while decreasing the burn duration. Make sure the air intake is clear of debris and away from hot engine areas like the exhaust manifold.
7. Increase Engine Speed
Short, slow speed urban jaunts don’t allow the engine to heat up and can cause carbon buildup. Low-speed knock may be caused by low octane fuel or old fuel from sitting in the drive for long periods. Ensure you’re using the recommended grade of fuel and take a run on the highway for an hour or so to clean out the pipes.
8. Replace Knock Sensor
If you hear engine knocking, it may be due to a faulty knock sensor. The sensor isn’t located in an easy-access location, and most people don’t have the equipment to check if it is working properly. Take your car to a trusted repair shop and have them check it out and replace it if necessary.
9. Make Mixture Richer or Leaner
A richer mixture has more fuel in the air-fuel mix, while a lean mixture has less fuel. The perfect mixture will burn all the fuel in the chamber and leave no oxygen. A rich mix will decrease knocking by lowering the compression temperature and reducing the incidence of premature detonation.
At higher altitudes, the air is thinner. Due to the thinner air, it takes a larger volume to burn a similar amount of fuel in the air-fuel mixture. A leaner mixture ensures a more complete burn in the combustion chamber and less chance of detonation knocking.
Can You Drive a Car With a Knocking Engine?
The first time you hear your engine knocking is when you should take action. The longer you delay, the greater the potential for damage to pistons, connecting rods, bearings, bushings, rings, and other things. The worst that can happen is a hole in the engine block requiring a whole new engine.
Valve and tappet knocking may only affect the performance and efficiency of your ride, for a while. Adding an extra quart of oil every now and then may delay the need for repair by days or even years. However, it’s best to repair a knocking engine before you find yourself walking.
There are many reasons an engine may produce a knocking sound, and all of them can cause damage. Changing the oil and spark plugs, using a higher fuel octane, or cleaning the engine may solve the noise. However, if the sound persists, it’s time for you to go knocking on your mechanics door.