You might be driving down the road and then all of a sudden hear a certain knocking sound coming from underneath the hood. More often than not, this relates directly to your engine. As we all know, the engine is one of the most important components of your vehicle – it is what keeps your car running smoothly and safely, and can require extremely heavy duty and expensive repairs or replacements if you let a problem with the engine go on for too long.
A combustion engine is supposed to run smoothly from the moment you turn your key in the ignition until you stop the car later in the day. However, sometimes an engine will churn out some noises that are a bit difficult to diagnose what exactly the root cause is. Sometimes, when you hear this strange noise coming from underneath your hood, you might not know what to do. We can help you.
Here we are going to figure out what the engine knocking sound is, the causes of engine knocking, the solutions of engine knocking, and how much you might expect to pay to fix the cause of the knocking sound.
What is engine knocking?
Knocking, also known as detonation, spark knock, or pinging, in spark ignition internal combustion engines occur when combustion of some of the air and fuel mixture does not create the necessary flame from the spark plug, but instead, one or more of these air and fuel pockets explodes outside of the normal area of combustion. The way it is supposed to happen is that the air and fuel mixture charge is meant to be ignited by just the spark plug at a certain point during the piston’s stroke. Knock occurs when the peak of the combustion process does not occur at the right moment in the four-stroke cycle, which is when an internal combustion engine completes four separate strokes while turning the crankshaft.
Why does this engine knocking sound occur?
The engine knocking sound occurs when there is abnormal combustion within the internal combustion engine. When unburned fuel and air mixture is exposed to heat and pressure for a longer time than normal, detonation may occur. This means that there will be an explosion of at least one pocket of air and fuel mixture outside of the flame front.
If this continues over several cycles within your engine, other parts of your engine can be severely damaged or even completely destroyed. The most common negative effect of continuous engine knocking is particle wear, causing erosion or abrasion in the engine. It can also cause failure by melting holes in the piston or cylinder head.
How to fix the engine knocking sound
Since we now know how serious engine knocking sound is in your car, it is nothing to ignore. Generally caused by a poor mixture of fuel and air that is required to power the engine, this causes the gas to burn unevenly and potential detonations to occur. If you run your car for too long and drive it for extended periods of time with this problem, serious damage could occur to the inner components of your vehicle.
In some cases, however, it is relatively easy to fix the engine knocking sound. Try these three things before bringing your car immediately to a mechanic.
- Only fill your car with premium unleaded fuel. The low-grade and cheap gasoline that you may be using every time you fill up your tank could be the real culprit. If you normally use this kind of unleaded gas, fill up with premium next time, since higher octane fuel can help fix your engine.
- Add fuel detergent. Although most gasolines and fuel mixtures contain some kind of fuel detergent, you might need something more pungent if you find that your engine is making a knocking sound. Adding the right kind of fuel detergent can help clean out the carbon that might be building up within the fuel lines, reducing the engine knocking sound.
- Replace worn out spark plugs. If your car has been serviced recently, there is a slim chance that the original spark plugs were actually replaced with the incorrect kind. They might still fire, but they don’t fire at the correct time. Make sure the right spark plugs are installed for your car to prevent the engine knocking sound.
Other reasons for the engine knocking sound
If you are hearing the loud knocking sound coming from underneath the hood, there are a few other problems that can cause what most people would describe as a knocking noise.
First, and the most common, is the engine knock. If you hear a rapid tapping sound coming from the engine as you drive your car, your engine is most likely suffering from knock. Paying attention to how you drive, you will notice that the issue gets worse when you accelerate. The engine knocking sound is a result of multiple detonations due to incorrect ignition timing, an incorrect air to fuel ratio, or a malfunctioning sensor.
Second, another reason for the engine knocking sound is rattling accessory pulleys. As the engine turns over, it runs a belt that rotates pulleys on various accessories under the hood to give them power. These accessories include the water pump, A/C compressor, power steering pump, and even more. If the pulleys become damaged, they might start to rattle while rotating. The higher the speed an engine spins at, the louder the noise will become.
Third, another reason for the engine knocking sound is a squeaking accessory belt. We already know that pulleys could be an issue for the noise, but the belt itself might have become stretched out or slipped out of place over time. This is especially prevalent when the engine is cold and you start the ignition after it sits for a long time.
Lastly, rod knock could cause the loud engine knocking sound. This is the loud clacking noise that you will hear as the driver if rod bearings become damaged over time. The piston rod is connected to the bearings, allowing the crankshaft to spin and the piston to travel smoothly. Damage to these bearings lets the rod knock against the crankshaft, since there is a small gap between the piston rod and crankshaft.
Causes of Fuel-Related Engine Knocking Sound
Fuel Has a Low Octane Rating
- If you put in fuel that has an octane rating that is too low for your specific vehicle, this can lead to the engine knocking sound. The octane rating relates to the fuel type’s ability to resist detonation before it is supposed to in relation to the air and fuel mixture within the engine. The combustion therefore causes a knocking sound. To keep this from happening, use gasoline with an octane fuel rating that is above what the manufacturer for your specific kind of car recommends. You can also try using an octane booster. This product helps to improve fuel compression in your engine before detonation, increasing the fuel efficiency and horsepower.
- Fuels that are given to your vehicles are supposed to have a detergent that helps clean out carbon and prevent it from getting stuck inside of your engine and fuel lines – but some may not. As fuel mixes with oxygen, it burns, and the residual carbon can form on valves, spark plugs, and other components that are directly involved in the ignition process of the fuel and air mixture. This reduces the volume inside of the cylinder and increases the pressure and compression.
Wrong Spark Plug Gap
- If you recently had your spark plugs replaced or you replaced them yourself and have realized they might not be the exact right ones for your vehicle, it can cause the engine knocking sound. The spark plugs have certain heat ranges, meaning that it can withdraw a certain amount of heat from the combustion chamber. With the wrong type of spark plugs, it won’t withdraw the right amount of heat and compression. In addition, the spark plug gap is the key place where the spark plug ignites the air and fuel mixture that powers the car. If the gap is too narrow, the spark might be too weak and too wide might prevent a spark from firing at all.
How to prevent Engine Knock
Detonation that occurs at the wrong point or too early in an engine is what spurs the engine knocking sound. Detonation can be prevented by following these specific techniques:
- Using a fuel with high octane rating, increasing the combustion temperature of the fuel and reducing the chances that it could detonate prematurely
- Enriching the air to fuel ratio, which reduces the combustion temperature and increases the chances of less of a chance of detonation at the wrong temperature
- Reducing peak cylinder pressure
- Decreasing the manifold pressure – the difference in air pressure between the engine’s intake manifold and that of Earth, which can be done by reducing the throttle opening
- Reducing engine load
- Slowing down ignition timing
If you employ one or more of these techniques, it could slow down premature detonation and prevent the engine knocking sound from occurring.
Since temperature and pressure are one and the same, knock can also be remedied by controlling the peak combustion chamber temperatures by reducing the compression ratio, employing emissions reduction techniques in petrol/gasoline and diesel engines, calibration of the engine’s ignition timing (the timing that is the release of the spark in the combustion chamber near the end of the compression stroke), and ensuring careful design of the combustion chambers and cooling systems.
Further, adding certain materials could also prevent the detonation from occurring and causing the engine knocking sound. Thallium is a chemical element that could prevent premature detonation, along with tetraethyllead, a soluble organic compound that can be added to gasoline to prevent detonation.
In addition, where you live can affect the prevalence of knocks in your vehicle. Knock is less common in cold climates, due to the lower peak temperatures and the lower combustion temperatures. Steam can suppress knock in hot temperatures to prevent higher than normal temperatures.
Along with temperature, the types of roads you drive on can impact the prevalence of engine knocking sound in your car. Turbulence has a huge effect on knock. Engines with good turbulence can knock less than other engines, with turbulence occurring when the mixture is compressed and burned.
Unfortunately, if you have a diesel engine, you are in a bit more of a predicament than if you have an engine that uses another fuel type. Since diesel engines have a fuel injected into highly compressed air towards the end of the compression stroke, there is only a short gap between the fuel being injected and combustion starting. The rapid increase in pressure and temperature can cause the engine knocking sound.
Lastly, the specific design of the injector pump, fuel injector, combustion chamber, and the pistons and cylinders can reduce the chances of knocking. Engines that use direct injection engines have higher instances of knock than an indirect injection, due to the lack of oxygen dispersal within the chamber and the higher injection pressures of fuel and air.
Engine Knock Fix Cost
If you have determined that using the strategies to fix the engine knocking sound and evaluating your car that the engine knocking sound is coming from a rod knock, you can expect this to actually be a hefty price tag in terms of repairs. An average job usually runs at between $2,000-$3,000, with more expensive and high performance vehicles costing almost double the repair cost.