Knocks in the Engine: How to Define and Fix the Issue?

Any extraneous knock that occurs when the engine is running can result in one of the most expensive breakdowns. Today, the Indy Auto Man mechanics offer the most common causes of knocking and explain what to do if such a sound occurs in the middle of the road.  

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This is a phenomenon in which the fuel in the cylinders burns incorrectly – untimely and at high speed. Essentially, it explodes inside the engine, creating shock loads on the pistons and other parts. Even in the cars with the most reliable engines, the detonation is accompanied by a ringing metallic sound. However, it also can happen with a faintly audible sound.

Such micro-explosions do not bode well. The engine temperature rises sharply, and prolonged detonation can cause the head gasket to burn out or the piston partitions between the rings to collapse. So the situation is serious.

Its cause is usually poor quality fuel, a lean mixture entering the combustion chambers, or engine overheating. Modern cars have a knock sensor that detects and records this unpleasant phenomenon. Then, the electronic control unit tries to correct the operation – for example, it ignites the gasoline a little later. But the sensor also does not last forever, and the ECU is not omnipotent, so the knock of a modern internal combustion engine may well be due to detonation.

What to do? To begin with, it is worth filling up with the highest quality fuel possible – for example, from network gas stations. And if there is a lot of bad gasoline in the tank, dilute it with the highest possible octane. Sometimes, this simple step solves the problem. If detonation is still perceptible, you should go to a service center for diagnostics. In older carbureted cars, adjusting the ignition angle can help.


Valve noise was once common in older engines. There are small gaps in the valve mechanism to compensate for the thermal expansion of parts. Over time, they increase, and the knocking of the valves intensifies.

The gaps in modern engines are better adjusted, and they have hydraulic compensators, which automatically react to fluctuations in engine temperature. The valves have become much less noisy, but this problem still exists. Usually, in such a situation, they say that the engine is clicking. It occurs due to a malfunction of the compensators, which operate due to oil pressure.

Violation of clearances is fraught with power losses. But the main thing is the failure of the valve and its seat (the place in the cylinder head to which the valve disc is adjacent). The problem is also dangerous to ignore it.

Faulty expansion joints require replacement. And on older cars, you will need to adjust the valves. If you have ignored the knocking noises for a long time, it is better to check the condition of the valves with seats. It can be assessed by the level of compression in the cylinders – if the valve drops quickly, it leaks.


If a high-mileage engine knocks, the cause may be wear on the pistons. In this case, they do not move along the cylinder smoothly and impact the walls at the moment of upward and downward movement. That is, it is not the rings that contact the cylinder walls but the piston itself. In this case, a knocking sound in the engine will be audible both at idle and in operating mode. The saddest scenario is the destruction of the piston, for example, due to the detonation. The knocking will become noticeably louder.

Piston wear, alas, cannot be treated in any way. The engine must be disassembled to carry out a major overhaul. And if the piston is destroyed, it is extremely undesirable to go somewhere further.

Lack of oil

What can knock in the engine if it is fully operational? Sometimes, sounds are not a consequence of wear or malfunction but a simple lack of oil. Then the knocking noise can come from various engine parts that lack lubrication: hydraulic compensators, phase shifters, and so on. It may be quiet, but that doesn’t mean it does not deserve attention. After all, a low oil level causes severe wear, so the engine life cycle dramatically reduces.

That’s why it is worth checking the oil level. If oil is insufficient, you need to top it up and find out why it suddenly dropped. The oil could have burned out or leaked through seals that had lost their elasticity. If there is very little oil, it is better to turn off the engine and not drive further – otherwise, you will not avoid the malfunctions with valves or pistons described above.

The faster you figure out the reasons for the knocking, the less risk of major repairs. If some strange sound appears, it is a fair reason to schedule a visit to a service station. 

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