The engine knocks after it warms up. What are the causes?

Engine knocks are always a sign of a problem that does require an immediate check. A motor that is knocking when it is hot can be for various reasons, hydraulic lifters, unregulated valves, or a bad timing chain and also cracked piston pin or skirt, bad connecting rod, or a failing bearing.

Whatever the problem is you should always check this problem in depth before things start to get worse and could lead to engine failure.

What are the causes of engine knocking when hot?

Typically, the engine is knocking because of the decreasing size of the gaps between engine parts inside. When the engine is cold, the oil is thicker, so the metal parts do not expand, as the engine gets warm, the oil becomes liquid and the gap of worn engine parts leads to knocking.

The main causes are:

1. Engine detonation.

The sound that detonation emits is similar to small muffled explosions in the cylinders. This is because fuel ignition is sudden and not slow.

2. A high mileage engine.

If it is above 110.000 miles, internal parts are worn out.

3. Crankshaft and its inserts.

Because the rods and main bearings are made of softer materials than the crankshaft, because of lubrication and service life, they can wear out, turn, and knock. When the rods wear out, a clanging sound appears. These sounds are heard only when the engine is warm. This may cause one, or more inserts to roll over and collide with the insert facing it on the crankshaft. As a result, the rod and piston can simply hit one another.
Wear of the liners is caused by insufficient oil level, carbon impurities, water, engine overheat, frequent high engine speeds. The maximum gap of engines differs on engines, but on average, is 0.07 mm.

4. Valves adjusted incorrectly.

Valves make clicking sounds when the engine load increases and when the oil level is low. Valves knocking are caused by worn valve rocker arms, a clogged camshaft oil channel through which oil is supplied to the valve rocker arm. The knocking is produced by the camshaft that hits the rocker arms that results in a pronounced metallic knock that repeats in time with the rotation of the camshaft. This happens only when the engine is hot.

Even if the engine oil level is insufficient, the idle valves may not knock without load. However, during acceleration, if there is not enough oil, you will hear a distinct, frequently repeated metallic sound.

5. Insufficient oil level in the engine.

This is the most often cause of engine knocking. Because the oil level is not enough in the engine, this can cause engine knocks because there is no proper lubrication. As the engine warms up, the oil dilutes and does not soften the impacts between worn parts so much.
If the oil level is insufficient, then the parts will interact “dry”, while making clanking metallic sounds. Low oil level is very harmful to the engine, it must either be completed until it reaches the desired level on the dipstick or replaced with another type of oil that suits the engine, along with the oil filter.

6. Worn hydraulic compensator.

Because of low-level oil in the engine, and not enough oil pressure, they can wear out. However this problem can be solved by simply changing the oil and the oil filter. Their failure is caused by:

  • bad oil pump;
  • low oil in the engine crankcase;
  • using an incorrect oil;
  • clogged oil filter;
  • adding extra additives that harm the composition of the engine oil;

7. Worn pistons and cylinder walls.

Because of a worn engine, the piston geometry will not be the same, as a result, the pistons and cylinder walls can fail. Overtime the piston skirt wears off and its diameter changes, the same is available for the cylinder walls in the block. When worn, they will emit a clanging metallic sound. A cracked piston ring, or rings, can also make a metallic knock when the engine is warm.

Depending on the damage, the sound can be heard when the engine is cold and when it’s warm, however, you will feel the difference in sounds, because when the engine is hot, the sound will be louder.

8. Bad crankshaft bearing.

As the engine wears out, this piece follows. The bearing can also be damaged by a bad repair in a workshop for example. The camshaft bearing fails more often on diesel engines.

The wear on the camshaft bearing is caused firstly, by the poor quality of the bearing itself, and, besides that, oil has an important role here. Being changed irregularly, being of poor quality, and being insufficient in the engine helps the bearing to wear faster. Frequent engine overheating and running with the engine at high speeds constantly can wear the bearing faster.

The crankshaft bearing can have knocks when the engine is cold and warm. In cold, the sound is only at the start and disappears after that. Even so, it is the first symptom, you can drive like that, but if the sound is heard when the engine warms up, you will have to change the crankshaft bearing.

How to determine what part is knocking in the engine?

To determine what part causes the knocks is difficult task. There are quite a few parts involved and sounds heard can mislead you. That’s why the best way to find out is to take the car to a professional car repair shop and make a detailed check to your engine.

The sounds that the engine emits are not always the same, they can be constant, occur at any time, can be loud or not. Usually, constant sounds are directly related to engine speed. The higher the engine speed, the more frequent are the knocks.

Frequent causes of engine knocks based on sounds that it emits.

Knocks when starting a warm engine:

  • crankshaft bearing;
  • turbo problems on diesel engines;
  • crankshaft liners.

The engine clincks when it’s warm:

  • incorrectly adjusted valves;
  • low engine oil level or the oil has already lost its properties;
  • phase shifters.

A thud noise on warm

  • engine detonation

A ringing metallic sound when the engine warms

  • worn-out hydraulic compensator;
  • crankshaft;
  • pistons or piston rings.

Knockings when idle

  • low oil level;
  • detonation;
  • hydraulic compensators.

Knockings when increasing speed

  • crankshaft liners;
  • incorrectly adjusted valves;
  • phase shifters;
  • pistons, cylinder walls, or piston rings;
  • crankshaft bearing;


Although some explanations are a bit “techy”, I hope you understood, at least just a bit, what the causes engine knocks when it is warm. These sounds are a headache for drivers, because they can lead to failures that require some money for repairing, and checking these knocks is not always that simple.

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