Why is the engine knocking when cold?

Any car owner is happy with the smooth, correct, almost silent operation of the car engine because this indicates its fully operational. But, various sounds that occur especially when the engine is cold can be alarming, because they are a kind of signal that indicates problems that require immediate elimination.

One of these signal is a knock in the engine when cold.

There are many reasons why the engine is knocking. Some knocks indicate only minor problems that can be easily diagnosed and corrected. But some sounds indicate problems with the internal combustion engine and require immediate intervention to avoid any unpleasant situations, such as engine overhaul.

Types of engine knockings

The occurrence of any sound is because one element of the internal combustion engine hits another, while each of the impacts may have its sound, intensity, duration, and frequency.

  • If the engine noise is weak, perhaps disappearing when driving, the car is usually not in danger of serious damage, but checking the engine timely is always a good idea even if there is no knocking 🙂
  • The average volume and intensity of the sound indicate more serious problems. In this case, the first thing you need to do is to go to the nearest workshop to avoid serious engine problems.
  • If the sound is clear and loud, then the driver should urgently turn off the engine and call a tow truck. If you continue to drive like this the engine may stall.

What are the causes of engine knocks?

There are quite a lot of reasons for the occurrence of knocks in the engine. The main ones are because of mechanical wear of parts and assemblies, as well as the formation of detonation processes. The nature of the sound depends largely on the cause of its appearance, that is, the failure of a part, or the degree of detonation. This factor becomes fundamental for a good diagnosis.

The knock that occurs when starting a cold internal combustion engine, followed by its warming up, can completely disappear, can become less intense, does not change its character during further operation of the engine, or might have higher intensity because of the increasing pressure and/or temperature.


So the main factors that can cause engine knocks are

  • Metal knocking in the engine provided that the internal combustion engine is fully serviceable, signals that the accepted norms of thermal gaps do not meet. You can correct this situation by adjusting the valves professionally.
  • The loss of performance of hydraulic compensators also causes the engine to knock when cold. Compensators fail, due to mechanical wear, the use of oil that is unsuitable for the car according to the required specifications, as well as its untimely replacement. To eliminate this problem, it is enough to perform a complete engine wash, replace the engine oil with a required one, and oil filter as well.
  • Increasing the gap between the main bearings may cause knocking in the first few seconds after starting the engine. It is self-canceling when the pressure indicator of the lubrication system increases. Incorrect operation of the bearings can also be caused by a decrease in the performance of the oil pump.

    In this case, the amount of incoming oil becomes insufficient, and at the same time, there is a blockage of the channels, which leads to the fact that the oil does not have time to produce high-quality lubrication of all components and parts. As a result, they begin to emit an unpleasant rattle and rasp.
  • Dull metal repeated blows in the engine when accelerating may indicate a failure in the connecting rods. This sound is characteristic of a worn connecting rod hitting the crankshaft neck. Here you need the urgent help of a professional car mechanic, as the operation of the car in this condition is strictly prohibited.
  • The characteristic sound in the engine also indicates that there are problems with the gas distribution mechanism. The most common reason is an increase in camshaft clearances. When the engine is subsequently warmed up, these knocks become either less intense or completely disappear.
  • If the piston is sufficiently worn, it can also make a tapping sound during operation. Damaged pistons move freely, as if “walking” in the cylinders, causing a characteristic metallic sound when the “skirt” hits. Similar knocking can cause the deformation of the piston rings.
  • The knock caused by the detonation is quite loud. Its intensity naturally increases with each engine cycle. The main reason is the untimely and uneven ignition of the fuel-air mixture, which provokes spontaneous destructive explosions inside the cylinders.
  • When the engine is cold, the appearance of detonation shocks, and hence the characteristic knock results in the use of fuel with lower octane (according to the manufacturer’s recommendations).
  • A very common factor in the appearance of engine knocking is the use of a thinner cylinder head gasket, which causes an increase in the compression ratio.
  • Various knocks in the under-hood area can be caused by various breakdowns of attachments and units, such as the water pump generator, timing, etc.

How to diagnose engine knocks yourself

If it is not possible to do a professional diagnostics in a car workshop, you can try to identify the reasons why the characteristic metallic sound appeared in the engine on your own.

  • Initially, you should clearly make sure that the sound comes from the motor, and not from other parts. If a manual transmission is installed in the car, it is recommended to fully depress the clutch to disconnect the clutch from the engine. In this case, it will be possible to determine that the cause of the knock is located in the engine, and not in the transmission.
  • Then you need to listen carefully to the knock, determine its duration, cycle, and intensity. As noted earlier, even a certain level of tonality is characteristic of different faults.
  • A loud sound that is gaining momentum and coming from the upper region of the cylinder head -> a problem with valve clearances;
  • A sound similar to the impact of a small metal ball on the lid with a characteristic build-up -> a malfunction of the hydraulic compensator;
  • Rustling, whistling, and creaking sound -> a bad or failing timing chain or timing belt, as well as the alternator belt;
  • Detonation has a ringing, bright sound, usually the term ” knocking fingers”
  • It is also mandatory to check the condition of mounting the engine supports and, if possible, the entire chassis
  • To listen more carefully to the engine sounds, car owners often use a technical type phonendoscope device. Professional diagnostics involves the use of an engine tester like OBD 2.


As you can see, diagnosing an engine knock is not an easy thing and it’s better to go to a professional mechanic that has all the tools needed for this operation

There are a lot of reasons for the appearance of noise in the engine, including knocks of various types. Many of them indicate minor problems with which the car can peacefully “coexist” for a long time. But also the knock that appears may indicate serious breakdowns of parts and/or engine components, which is strictly forbidden to ignore.

The design of the engine implies the interaction of many parts and components that are subjected to quite significant loads during operation. A marker for many internal combustion engine failures is the outgoing knock, which has its own tone, duration, and intensity for each type of failure.

Thanks to this, with careful diagnostics, you can not only identify the cause of the engine malfunction, but also eliminate it in a timely manner.

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