Have you noticed an engine knocking sound and you are freaked out why there is knocking sound under car when accelerating? Well, if that is the case, then you are at the right place because there will be quite a lot to cover on this topic.
Noticing weird sounds and noises when the car is working should be one of your top priorities. Why I’m saying this? Well, because it is true. Catching a problem on time could save you thousands of dollars in the long run.
There could be some really bad problems that could basically scrap the whole engine altogether if you are unaware and you act sluggish. When you hear something strange, you need to act quickly.
First, learn more about the problem and the possible causes, and then head up directly to the mechanic if you do not have the right tooling to investigate the matter. But you shouldn’t worry because we will give our best to help you out when it comes to this problem and learn if this is something really scary or something not so frightening.
First, we are going to elaborate more on the problems of what knocking sound under car when accelerating is. And learn about if this problem is serious and how you should handle the issue. Then we will cover the most probable causes for the knocking sound and how you can diagnose the problem at home. And also, how much it will cost you to tackle the problem? So, if you want to learn more about this problem and how to solve it, follow along.
Engine Knocking Sound
Now before we dive into the causes of the knocking sound, we need to elaborate more on this problem first and learn more about it. So, if you are new to this problem, this will be a nice introduction for you.
So, what is knocking sound under car when accelerating? Well, this sound is audible information that something is wrong with your car. Which is obvious.
But when it comes to the location of the sound is really important to learn where exactly this sound comes from.
Since the engine bay is really large. You don’t know what could it be unless you test things out and troubleshoot the problem.
You can learn the possible causes which we are going to cover later on in this article. But you need to learn how to perform proper diagnostics.
You need to track that sound and learn the exact location of it if you want to sort out the problem with knocking sound under car when accelerating. But you shouldn’t worry because later on in the article, we are going to cover all about this and learn how to diagnose knocking noise when driving and knocking noise in the engine.
The important thing in this situation is not to panic because if there is damage, it is already done. You can make it worse if you keep on driving the car that is for sure. But it probably won’t be much unless you have rod knock symptoms. And more about rod knock and the danger we will see later on.
Is This Knocking Sound When Accelerating Serious
Another question that people like to ask when they hear a knocking sound under car when accelerating is the level of concern that they should put into this type of problem. Is this problem serious or not?
Well, the level of concern really will depend on the place where this sound comes from. If the sound comes from the wheels, I would get concerned because the wheels keep you on the road and make sure that you don’t crash your vehicle into something. Also, the steering system here plays a big role since the steering rack is right below the engine.
Or if the sound comes directly from the engine, then probably there is something going wrong inside it. There could be timing-related problems, problems with ignition, and also problems with piston rods.
So, you really need to be aware of these issues and act accordingly when you face some of these symptoms like the knocking sound under car when accelerating.
Mainly because when the car is stationary it is not quite noticeable. But the sound starts only after you apply the foot on the gas pedal and try to drive off. So, this is a crucial aspect that narrows down a little bit of the possibilities when it comes to the problem itself.
But you shouldn’t worry, because, in the following chapters, we are going to cover all of the possibilities for the knocking sound under car when accelerating and we will learn everything that could cause this sound.
And trust me, there are a lot of possibilities for this problem. There are so many that from now on, right before we end we will only discuss the causes for this knocking sound under car when accelerating. So, you definitely don’t want to miss that.
Engine Knocking Causes
So, we elaborated more on this problem when you notice a knocking sound under car when accelerating. Now let’s take a look more at the causes that can trigger a problem like this.
And honestly, this is a really broad problem that can have a dozen of possibilities. This is why we mentioned previously that you should narrow down a little bit the possibilities by investigating deeper into the matter and manage to catch where exactly this sound is coming from.
But anyways, we are going to cover all of the possibilities in the following chapters when it comes to the knocking sound under car when accelerating and will try to find where this sound comes from. So, if you want to learn more about the engine knocking causes, follow along.
Knocking Sound Under Car When Accelerating, Causes #1: Low Octane Gasoline
Now let’s dive into the causes of knocking sound under car when accelerating. The first in our list and probably the most common cause is the low octane gas. So, why is this the case? Why would low-octane gasoline do this to your vehicle and ruin its performance?
Well, low octane gasoline has a bad effect when poured into some higher compression engines. If you didn’t know, the octane number is the ability of the gas to resist compression. So, when you have low octane gas, it performs quite poorly.
It will detonate inside the cylinder even before the end of the stroke. Causing uneven explosions inside of the cylinder.
So, if the car needs a 92 octane gas, you better add this type of gas if you don’t want to hear a knocking sound under car when accelerating.
This condition is also known as a piston slap. And it is not dangerous at all. Whenever you added this type of gas into your car, just make sure that you add premium gas next time and you will be good to go.
Always pour the recommended type of fuel and if your car does not like the gas from that specific pump, just change the gas station. Sometimes changing the gas station helps sort out these types of problems. Now let’s move on to the next cause of knocking sound under car when accelerating and knocking noise when driving.
Knocking Sound Under Car When Accelerating, Causes #2: Bad Knock Sensor
Now let’s move on to the second probable cause for knocking sound under car when accelerating. And this cause is the knock sensor. But what are the bad knock sensor symptoms?
Well, if you are not familiar, a knock sensor is a sensor that is screwed into the engine block. This sensor is basically a microphone that is fine-tuned to record sounds on some frequencies.
It records knocks happening inside of the engine. These knocks might be knocks because of a bad octane rating as we mentioned previously. They also could be because of poor timing of the camshaft, or possibly because of a rod knock.
So, this sensor records these sounds, and then the computer adjusts the air-to-fuel ratio of the car. Or possibly shows the check engine light if there is a problem.
This sensor is essential in order for you to avoid having an engine that produces knocks. So, if the sensor fails, the engine could start knocking more often. It could go out of timing and in the long run, it could destroy itself. So, checking this sensor’s health is crucial sometimes to sort out the knocking sound under car when accelerating. Now let’s move on to the next probable cause.
Knocking Sound Under Car When Accelerating, Causes #3: Bad Spark Plugs & Ignition Coils
The next cause that we are going to cover for the knocking sound under car when accelerating is concerned with the ignition. More precisely the spark plugs and ignition coils.
When the ignition components fail, they start to create misfires in the cylinders. So, what is a misfire?
Well, a misfire technically is a missed combustion. The combustion is not happening when it should. It is either premature as we mentioned previously with the octane knock. Or possibly delayed because of poorly working spark plugs and ignition coils.
So, whenever this happens, it could cause the engine to knock and there will be a knocking sound under car when accelerating. In these situations, it is important to notice this quickly and uncover the problem because if you don’t, it could be damaging to the engine. Now let’s move on to the next probable cause for the knocking sound under car when accelerating.
Knocking Sound Under Car When Accelerating, Causes #4: Lean Air To Fuel Mixture
The next probable cause for the knocking sound under car when accelerating is the lean air-to-fuel mixture. So, what is a lean air-to-fuel mixture and how does it affect the engine performance?
Well, a lean air-to-fuel mixture basically is the situation when you have more air in the system than fuel.
The ideal air-to-fuel ratio should be 14.7:1. Which means 14.7 grams of air, per gram of gasoline. Whenever there is more air than recommended, there is something called a lean state.
And this is a situation that you want to avoid. Whenever there is a lean state, there will be knocks inside of the piston, and a knocking sound will be present under acceleration. So, what is causing this problem?
Well, the reasons behind this problem can be a few. But the most notable reason is a bad MAF sensor, a bad MAP sensor in some cars. Or possibly a vacuum leak. Vacuum leaks can come from the intake manifold, the PCV system, and the EVAP system.
And finding vacuum leaks can be a real nightmare if you are asking me. The best thing is to locate them with a smoke machine. Otherwise, you will be only left guessing. Now let’s move on to the next probable cause for the knocking sound under car when accelerating.
Knocking Sound Under Car When Accelerating, Causes #5: Bad Engine Timing
Let’s now move on to the next probable cause for the knocking sound under car when accelerating. And this is the case when you have bad engine timing. So, what is engine timing?
Well, engine timing basically is the position of the camshafts and the position of the crankshaft. Proper engine timing needs to be in the engine unless you want problems to appear.
Mild problems can include misfires and knocking noises. While more extreme issues with timing can be bent valves and damaged pistons.
When it comes to the causes of bad engine timing, there can be a few. Bad timing components. Timing belt or chain and their tensioners respectively. Then there could problems with the variable valve timing of the engine if the engine has one.
Also, problems with the crankshaft and camshaft position sensors. These sensors are the ones that monitor the position of the camshaft and crankshaft. Overall, there could be many reasons for bad engine timing. Just beware of this. Now let’s move on to the next probable cause of the knocking sound under car when accelerating.
Knocking Sound Under Car When Accelerating, Causes #6: Rod Knock
Now let’s move to one of the most common causes of the knocking sound under car when accelerating. And this is the rod knock. So, what are rod knock and rod knock symptoms?
Well, this is the type of problem that you don’t want in your engine. The rod knock is something really expensive to fix because in most cases, the whole engine has to be replaced. But why does it happens?
Mostly because of negligence by owners. They don’t change the oil regularly. And the bearings on the crankshaft wear down. And once they wear enough, there will be a slight imbalance of the pistons. So, the rods will start to knock against the crankshaft.
In these situations, besides the rod knock symptoms, big engine damage will be present. The crankshaft will get scratched, cylinder walls will get scratched. Basically, a nightmare to fix. So, the best is to replace the whole engine when you have a rod knock. Now let’s move to the next probable cause for the knocking sound under car when accelerating.
Knocking Sound Under Car When Accelerating, Causes #7: Bad Engine Mounts
Now let’s cover some causes for the knocking sound under the car when accelerating that are not concerned with the engine.
The first on our list is the bad engine mounts. Engine mounts can fail. These mounts are basically the buffer between the engine and the chassis. And the more aggressive the damage to the mounts, the worse the sounds will become.
You can diagnose bad engine mounts if rev the car and see if the engine wobbles around. If it does, the mounts are probably bad.
Knocking Sound Under Car When Accelerating, Causes #8: Bad Catalytic Converter
The next in our list of causes for the knocking sound under the car when accelerating is the bad catalytic converter (it might help to know the catalytic converter life expectancy). So, why is this a possibility?
Well, this is possible because, inside the catalytic converter, there is a honeycomb structure (to learn more, check out our guide on what do catalytic converters do and what does a catalytic converter do). And if the converter gets clogged. This structure can disintegrate.
And when it cracks into small chunks, it will end up hitting the walls of the catalytic converter. And the faster you go, the stronger the sound will be. So, beware of this problem because it is a possibility.
Knocking Sound Under Car When Accelerating, Causes #9: Bad U-Joint
Now let’s cover the last probable cause for the knocking sound under car when accelerating. And this is the bad U-joint. So, what is a U-Joint?
Well, U-joint, also known as a universal joint is an integral component of the driveshaft of the rear-wheel drive car.
This shaft goes in the center of the vehicle from the engine towards the rear differential. This component has U-joints at both ends. And what can happen is that these joints can fail and when they fail, they will start to produce a knocking sound under car when accelerating.
And probably these are the reasons why you are having this problem. But how you can diagnose a problem like this on your car? Let’s find out more about that next.
How To Diagnose Knocking Sound Under Car When Accelerating
Now let’s dive into diagnosing the knocking sound under car when accelerating. How you can diagnose this sound and learn more about where it is coming from?
First thing first, you need to learn from what part of the car the sound is coming. Whether it is the engine or the center of the vehicle where the driveshaft is located. If it comes from the center tunnel, it is important to check the U-joints and see the condition of the driveshaft.
The next thing you want to check is the engine mounts. Make sure that the engine does not move a lot under heavy load. If it moves around, then probably the mounts are broken.
Also, check the catalytic converters and make sure that the sound does not come from them. If everything we mentioned is ok, then you will need to move on to inspecting the engine.
Check if there is a rod knock, you can see videos on Youtube about how a rod knock sounds. Also, check the knock sensor, check your fuel, and see if it’s low octane. Sometimes the answers to these problems can be found in the little things.
As we mentioned, the problem is too broad and we cannot precisely tell you what to do, so you will need to investigate based on our leads.
Knocking Engine Fix
When it comes to the cost to fix the knocking sound when accelerating, we can say that this problem could be something really small that does not cost much.
Or a bigger problem like a rod knock that will basically scrap your whole engine. And a new engine will cost you $3,500 in the best case and up to $7,000 if it’s some more modern car. So, this is why you should invest more time into solving this problem and making sure that you diagnose it correctly. Don’t delay stuff, the more you delay, the more expensive the repair could become.
In this article, we have covered the problem of knocking sound under car when accelerating. First, we learned more about the problem and learned if it was serious.
Then we covered the causes of knocking sound under car when accelerating. We elaborated on most of the possibilities. Then we shared some diagnostic tips that you can try as well as the possible cost of fixing the problem.
Now let’s answer some frequently asked questions.
What Is Rod Knock
Rod knock is a situation where the piston rod of the engine is hammering the crankshaft. This is because the rod bearing that sits between these two components is worn out.
What Causes Rod Knock
Bad rod bearings are causing rod knock. And bad rod bearings are often the consequence of poor maintenance. These bearings simply wear down and the rod starts to knock against the crankshaft. So, the only option to sort out this problem will be an engine rebuild.
How To Fix Engine Knocking
This depends on the problem. If it’s a rod knock, the only way to fix the problem would be to get a new engine. Or rebuild the existing one if it’s rebuildable. So, it really depends on the extent of the damage.
What Does A Rod Knock Sound Like
When you have a rod knock you will hear metallic sounds knocking inside of the engine. These sounds will come from the bottom of the engine. And the more throttle you apply, the stronger the sounds will become.
Why Is My Car Ticking
When an engine is ticking, you probably are having a misfire. When you have a misfire, the engine produces ticking noise. This can often be attributed to bad spark plugs or ignition coils. But there could be other problems like low octane gas, and a bad knock sensor.
What’s A Knock Sensor
A knock sensor is a small microphone that is installed on the engine block. This sensor records sound at specific frequencies and notices whenever there is knocking noise when accelerating and tells the computer to adjust the engine work in order for the engine to avoid damage.
These tools have been tried and tested by our team, they are ideal for fixing your car at home.