Knocks in the engine occur when parts of the crank mechanism are worn. They listen and find the place of knocking with the help of stethoscopes. The simplest stethoscope is a metal rod with an earpiece. By applying the tip of the rod to various points of the block or cylinder head, the cause of the knock is determined by the characteristic shades of the sound and at the place of its occurrence. There are also electronic stethoscopes, consisting of a low-frequency transistor amplifier and a piezoelectric sensor.
Determining engine malfunctions by knocking requires a lot of skill. In addition, to eliminate malfunctions of the crank mechanism, it is required to remove the engine from the car and perform its complete or partial disassembly. Therefore, in the event of knocking or any other malfunctions of the crank mechanism, it is recommended to contact a service station, where experienced specialists will determine and eliminate the malfunction.
compression (pressure) in the engine cylinders is checked with a special device - a compression gauge. It is a pressure gauge with a check valve. To measure compression, install the tip of the compression gauge in place of the turned out spark plug and, turning the crankshaft with a starter, record the maximum pressure in the cylinder using the pressure gauge.
Knocking in the main bearings of the crankshaft
This is usually a low-pitched, metallic thud. It is audible at the bottom of the cylinder block and is detected when the throttle is suddenly opened at idle. Excessive crankshaft clearance causes a sharper knock with uneven intervals, especially noticeable with a smooth increase and decrease in the crankshaft speed.
Causes of knocking and how to eliminate it:
- too early ignition. Check and adjust the ignition timing;
- insufficient oil pressure. See chapter «The main malfunctions of the lubrication system»;
- increased clearance between the crankshaft journals and the main bearing shells. Contact a service station to check and, if necessary, to regrind the necks and replace the liners;
- increased clearance between the thrust half rings and the crankshaft. With the engine off, check the axial free play of the crankshaft by depressing and releasing the clutch pedal. In this case, the movement of the front end of the crankshaft should be no more than 0.35 mm. In case of greater axial free play, contact a service station to replace the crankshaft thrust washers.
Knocking rod bearings
Usually the knock of the connecting rod bearings is sharper than the knock of the main bearings. It is heard in the upper part of the cylinder block at idle speed of the engine with a sharp opening of the throttle. The place of knocking is easy to determine by turning off the spark plugs in turn.
Causes of knocking and how to eliminate it:
- insufficient oil pressure. See chapter «The main malfunctions of the lubrication system»;
- excessive clearance between the crankshaft journals and bearings. At a service station, grind the crankshaft journals and replace the liners.
The sound of pistons and piston pins. The knock of the pistons is usually unvoiced, muffled, caused «beating» Piston in Cylinder: It is best heard at low crankshaft speed under load. The knock of the fingers is distinct and sharp, intensifies with an increase in the crankshaft speed and disappears when the cylinder is turned off from work. Audible at the top of the cylinder block.
Causes of knocking and how to eliminate it:
- increased clearance between pistons and cylinders. Repair the engine by boring and honing the cylinders and replacing the pistons;
- excessive clearance between the piston rings and the grooves on the piston. Replace rings or piston with rings;
- excessive clearance between the pin and the bore in the piston. Replace piston and pin.
Low compression in cylinders (engine does not develop full power). Compressometer shows the amount of compression in the cylinder is less than 10 kgf / cm2. Pour into a cylinder with reduced compression 20... 25 cm3 clean engine oil and check the compression again. In this case, two cases can be observed.
A) The compression gauge gives a higher compression reading.
Causes of the malfunction and how to fix it:
- breakage or occurrence of piston rings in the piston grooves. Clean the rings and grooves of the pistons from carbon deposits, replace the damaged rings and the piston;
- excessive wear of cylinders and piston rings. Repair the engine by reboring and honing the cylinders and replacing the pistons.
b) Compressometer readings do not change. Set the piston to the V.M.T. of the compression stroke, brake the car with a parking brake, engage the highest gear and supply compressed air at a pressure of 2... 3 kgf / cm2 into the spark plug hole2. This will cause air leakage.
Causes of the malfunction and how to fix it:
- poor fit of valves to the seats. Air is leaking into the carburetor (inlet valve leaking) or muffler (exhaust valve leak). For cause and remedy, see chapter «The main malfunctions of the gas distribution mechanism»;
- blown cylinder head gasket. Compressed air with a characteristic hissing sound passes into the adjacent cylinder. Remove cylinder head and replace gasket.
Oil getting into the coolant
There is a decrease in the oil level in the engine, an oil film appears in the expansion tank, the color of the coolant changes from gray to dark brown.
To check, remove the cylinder head, fill the cooling jacket of the cylinder block with water and apply compressed air to the vertical oil channel of the cylinder block (near bolt hole 5, see fig. 22). If air bubbles are observed in the water filling the cooling jacket, then the causes of the malfunction are shells or cracks in the jumpers between the oil line and the cooling jacket of the cylinder block. In this case, the cylinder block must be replaced.
If the oil passages in the cylinder block are tight, then oil may be entering the coolant from the oil passages in the cylinder head. In this case, it is necessary to check the tightness of the cylinder head (see chapter «The main malfunctions of the gas distribution mechanism»).
Coolant getting into oil
The liquid level in the expansion tank is constantly decreasing, and the oil level is increasing. The oil changes color from gray to milky white.
The causes of the malfunction are shells, porosity or cracks in the walls of the cooling jacket of the cylinder block. To check this defect, it is necessary to disassemble the engine and check the tightness of the cooling jacket of the cylinder block in a bath of water, supplying compressed air to the jacket at a pressure of 2... 3 kgf / cm2.
If air etching is not observed, then it is necessary to check the tightness of the cylinder head (see chapter «The main malfunctions of the gas distribution mechanism»).