Every motorist occasionally has to hear knocks while the engine is running. What causes these sounds and is it possible to determine by ear which particular part is out of order?
Common reasons
As a rule, knocking in the engine during normal lubrication and cooling occurs due to the fact that between the parts in the joint, the gap increases. Usually this sound is audible when the gap becomes about three times the nominal value. As the gap increases, the wear rate of the engine parts increases, and the knock becomes louder.
Sometimes a situation arises when a knock in the engine occurs even with a normal amount of clearance. In this case, it can be caused by such factors as heavy loads on the engine, skewing and jamming of one of the parts, decrease in oil viscosity due to overheating or dilution with another liquid (for example, when fuel enters the oil).
Determined by sound
We make a reservation right away - it is sometimes very difficult even for a minder with experience to determine knocks in the engine by ear. But general laws still exist.
If the knock appears regularly, then it is worth testing the engine in different modes. As you know, the parameters of the engine are the crankshaft speed and load, which increases as the throttle opens . If the sound amplifies with an increase in the engine speed, then most likely the problems should be sought in the piston group and the gas distribution mechanism.
With an increase in the load, the pressure in the cylinders increases and, accordingly, the load on the moving parts of the engine - primarily the piston group - increases. Therefore, in the presence of defects in the crank mechanism, the knock in the engine increases as the load increases.
Now let's try to determine the “knocking” part by the key of the knock. If when the engine is idling, you hear a thud, which then, as the number of revolutions increases, becomes louder and more often, then this “knocks” the crankshaft, in which the liners of the connecting rod or main bearings or the shaft journal are worn. The reason may be an untimely replacement of the oil or oil filter.
A thud in the engine “on cold”, that is, when starting a cold engine, may indicate wear on the camshaft bearings. As a rule, such a knock lasts no more than a couple of seconds until the oil gets on the bearings. If then the knock disappears, then with such a camshaft you can still drive up to 50,000 km. Later, the knock becomes more audible even when starting a warm engine, and inattention to the engine leads to increased fuel consumption, deteriorated starting qualities of the power unit, and deposits are deposited on the valve seat. The cause of the wear of the camshaft bearings may be poor oil quality, the operation of the bearings when there is a lack of oil or lack thereof, water or impurities getting into the oil.
Light metal knocking in the engine, which occurs when the speed increases, may indicate detonation. This is the name of the phenomenon in which fuel burns hundreds of times faster than under normal conditions. During detonation, a shock wave arises, which during the movement of the piston down dozens of times is reflected from the piston and cylinder head, causing the same knock. In addition to knocking, a drop in engine power, shaking of the unit during operation, and overheating of the cylinder head can indicate detonation. The cause of detonation is the use of low-octane fuel, the formation of soot in the combustion chamber, the failure of the exhaust gas recirculation valve. The result of detonation is damage to the pistons and piston rings.