Warm up the engine. When the oil is warm, the piston rings have a better seal.
On injection engines, disconnect the fuel pump relay. Otherwise, the engine will flood with fuel during the compression test, which can damage the catalyst. "Book" ignition.
Remove all spark plugs.
Press the rubber cone of the compression gauge into the hole for the spark plug of the 1st cylinder (first in direction of travel) and screw the connector into the spark plug thread.
Comment. The connection of the measuring device must be absolutely tight.
On diesel engines, instead of glow plugs, screw in a compression gauge, having previously disconnected the wire from the injection pump solenoid valve.
Apply the handbrake, the shift lever should be in the neutral position. Put the automatic transmission selector in position "R".
Have an assistant turn the engine over with the starter. The accelerator pedal must be fully depressed (for better cylinder filling).
When the pressure value ceases to increase significantly, record the measured value and move on to the next cylinder.
Required amount of compression (gasoline engines): 9-14 bar.
Minimum compression value: 7 bar.
Maximum pressure difference between cylinders: 3 bar.
Compression pressure too low
Low but even compression is not always a cause for alarm - differences between individual measuring instruments may be the cause. A more significant cause for concern will be if, after measuring on four or five-cylinder engines, the difference between the individual cylinders is more than 3 bar. This could mean the following:
Wear of pistons and piston rings.
Jamming of the piston ring due to the formation of soot.
Deposits on valve stems or seats due to carbon deposits.
Burning valves due to small valve clearance. In most cases, valve leaks are the cause of insufficient compression, resulting in a loss of engine power. The solution is either grinding the valves or repairing the cylinder head.
In order to localize the malfunction if the compression is too low, it is necessary: to drop a little engine oil into the spark plug hole and measure the compression again.
If the compression pressure is still bad, then the cause is in the valves.
If the pressure increases, then the cause lies in the piston rings and, possibly, in the cylinders. The introduced oil can temporarily improve the tightness of the piston to the cylinder walls, so that the compressible gas will not find an outlet.
Attention. The compression ratio should not be confused with the compression ratio The compression ratio is determined by design features, such as the piston stroke and combustion chamber volume. In addition, the compression amount and compression ratio may vary depending on the condition of the engine.
Pressure loss test
More accurate information can be obtained by performing a pressure loss test. The measuring device consists of two chambers, in one of which the pressure is the same all the time. The second chamber is connected by a hose through the spark plug hole to the combustion chamber. Through the nozzle, a connection is made with the first chamber and with the measuring scale.
If the pressure drops in the tested chamber, this is indicated on the scale. A serious leak can be identified by hearing:
- A hissing sound in the exhaust tract indicates a leaky exhaust valve.
- If compressed air comes out of the air filter housing, then there is a defect in the intake valve.
- If there is a defect in the cylinder head gasket or a crack in the cylinder head, pressurized gas will escape from an adjacent spark plug hole or from an open coolant expansion tank.
- When the cylinder walls, working surfaces of the pistons or piston rings are worn, gases enter the engine crankcase and exit through the open oil filler neck or through the oil dipstick hole.
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