A knock in the motor: causes and consequences


2020-01-14 09:20:08




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Every motorist from time to time we hear knocking during engine operation. What causes these sounds and whether it is possible to determine by ear what kind of item out of order?

Common causes

Usually a knock in the motor under normal lubrication and cooling arises from the fact that between the parts in connection, increased clearance. Usually this sound can clearly be heard when the gap is about three times larger than the nominal value. As soon as the gap is increased, the rate of engine wear increases and the sound becomes louder.

Sometimes, the situation where knocking occurs in the engine during normal clearance. In this case, it may be caused by such reasons as large engine load, misalignment and binding one of the components, reduction of oil viscosity due to overheating or dilution by another fluid (for example, when the fuel gets into the oil).

Define the sound

Say at once – to determine the knocking in the engine by ear is sometimes very difficult even for mechanic with experience. But the General laws still exist.

If a knock appears regularly, you should test the engine on different modes. As is known, the parameters of operation of the engine are the engine speed and load, which increases with throttle opening. If you increase the engine rpm the sound increases, it is likely that the problem lies in the piston group and gas distribution mechanism.

With the increase in load increases the pressure in the cylinders and, consequently, increases the load on the moving parts of the engine – primarily the piston. Therefore, in the presence of defects in the crank mechanism in the engine knock increases as load increases.


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Now let's try to define “knocking” detail key knocking. If when the engine is idling you hear a thud, which then, with increasing speed, it becomes louder and more frequently, this “knocking” crankshaft, in which worn liners connecting rod or crankshaft bearing or shaft journal. The reason could be failure to change oil or oil filter.

A thud in the engine «cold», that is, when starting a cold engine, it may indicate deterioration of the bearings of the camshaft. Typically, such knock lasts no more than a couple of seconds until the oil gets to the bearings. If the knocking then disappears, then the camshaft is still possible to travel up to 50,000 km Later the knocking becomes more audible even when starting warm engine, and inattention to the engine leads to the fact that increases fuel consumption, worsens startability of the power unit, as well as the deposition of carbon on the valve seat. Cause wear of the camshaft bearing can be a low quality oil, the bearings for lack of oil or lack of it, getting into the oil or water impurities.

Light metal rattling in the engine that occur when the revs increase, may indicate detonation. The so-called phenomenon in which the fuel burns hundreds of times faster than under normal conditions. When detonation occurs the shock wave, which during the movement of the piston down dozens of times is reflected from the piston and cylinder head, causing the very sound. In addition to knocking, detonation may indicate a drop in engine power, shaking of the unit during operation, overheating of the cylinder head. Cause of detonation is the use of low octane fuel, carbon buildup in the combustion chamber the failure of the EGR exhaust gases. The result of the detonation becomes damaged pistons and piston rings.

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