19.2.
Cylinder Compression Test
The compression test measures the pressure developed by the up ward movement of the piston on its compression stroke. The amount of pressure produced by the piston depends on the amount of air present in the cylinder, the compression ratio of the cylinder, and how well the cylinder is sealed. As the piston moves upward, the volume of the cylinder is reduced, and this increases the pressure of the air contained in the cylinder. Prior to the compression stroke, the pressure of the air in the cylinder was at atmospheric pressure. With a compression ratio of 8:1, the pressure is increased to more than eight times atmospheric pressure or more than 811 kPa. The pressure greater than 811 kPa is caused due to the increase of temperature as the air pressure increases. This temperature rise can cause the pressure to increase about another 172 kPa, thereby producing a total pressure of about 983 kPa.
The expected cylinder pressures of an engine are usually indicated in the specifications for the engine. If these specifications are not available, the following formula can be used to determine the value of pressure expected from the cylinder during the compression stroke;
Compression pressure = atmospheric pressure times the compression ratio plus
atmospheric pressure plus 69 kPa = 101.4 kPa x 8 (8 for a ratio of 8:1) + 101.4 + 69 kPa
= 981.6 kPa.
A compression tester is used (Fig. 19.9) to measure the compression pressure in a cylinder. The tester is inserted into the spark plug hole of the cylinder and the pressure of that cylinder is registered on the gauge of the tester. The pressure, acting on a diaphragm inside the gauge, moves the needle of the gauge to indicate the pressure on the diaphragm. The compression tester is fitted with a one-way valve, which allows the pressure to build up in the tester to determine the maximum pressure produced by the cylinder.
Fig. 19.9. Common types of compression gauges.
To use the tester, run the engine until it is warm. Then remove all the spark plugs from the cylinders. Now lock the throttle plates and the choke plate in a wide-open position. This permits
a maximum flow of air into the engine cylinder. The ignition should now be disabled, for which, disconnect the battery lead to the coil. The tester should be placed in the spark plug hole (Fig. 19.10) of one cylinder. The tester threads into the plug hole.
Fig. 19.10. Compression gauge in place.
With the tester connected to the cylinder, crank the engine through four complete compression strokes. At each compression stroke you may notice that the needle of the gauge jumps to a higher reading. The reading of the fourth compression stroke is the reading that has to be compared with the specification and to the other cylinders. Take a compression test of each of the cylinders in the engine and record the final reading of each. While the engine is rotating through the four compression strokes, the gauge reading should increase with each stroke. If the gauge does not increase with each compression stroke, this indicates that the valves of that cylinder are sticking. Sticking valves are due to carbon or gum deposits built on the steam of the valves, which cause them to resist closing or opening.
Compare your findings with the specifications. If the readings of one or more cylinders are below the specifications, there is a problem with the valves, piston rings, or the head gasket (Fig. 19.11). Any reading lower than the
specifications indicate that the cylinder is not producing as much pressure as it should. However, a small difference between the readings and the specifications is not a cause to be concerned. Any engine, no matter how well it is maintained, experiences some wear. A reading that is less than 75% of the specifications is too low. More important for diagnosis is the avenues of the readings among all the cylinders. If the cylinders have about similar readings that are low, the engine is simply worn evenly. If one or more cylinder readings are low and the others normal, this unevenness indicates that the compression losses in that engine cause it to run roughly. Low compression readings
Fig. 19.11. Areas of potential compression leaks.
indicate that the valves are not sealing because they are burned or the valve stems are bent, the piston rings are worn, or that the head gasket is leaking.
To identify the cause of low compression, two further steps can be followed. If the readings of two cylinders, located next to each other in the engine, are low, then it indicates that the head gasket is leaking. Usually, the area between cylinders is small and the head gasket is responsible for the separation of the cylinders. If the head gasket is defective, the compressed air from one cylinder can leak into the cylinder next to it through the leak in the gasket (Fig. 19.12). This is a quick method of finding the condition of the head gasket.
Low readings can also be caused by defective valves or worn piston rings. To determine the cause for the low readings, squirt a small amount of oil into a cylinder, which has low compression. With the compression tester, retest that cylinder. If the reading is higher than the previous reading, bad rings are the cause. The oil put into the cylinder temporarily seals the piston ring to the cylinder walls, causing an increase in the readings. If the reading remains almost the same, the cause of the low readings is the valves. Valve seals become lose due to burning away of the metal of the seat or of the valve itself. Oil does not provide a seal for the valves if they are burned. A compression test conducted with oil in the cylinder is called a “wet” compression test.
High compression readings can also result from the build up of carbon on the piston top and in the combustion chamber. As shown in Fig. 19.13, this build-up decreases the volume of the chamber, which in turn increases the compression ratio of the engine. Higher compression ratios give rise to higher compression readings. Therefore, high readings may also indicate that there is an excessive amount of carbon in the cylinder.
Fig. 19.12. Compressed air leaking into the cylinder next to it through a defective head gasket.
Fig. 19.13. Two identical cylinders, one free of carbon build-up and the other with considerable carbon build-up.
A carbon build-up gives high readings on the compression gauge. The wear of the valves and piston rings cause a decrease in the pressure readings. The combination of these increase and the decrease readings may balance each other and give a normal reading, which may not be a true4ndication of the condition.
However, the compression test is very important in diagnostics. If the compression of some cylinders is lower than the rest, the engine runs rough. To correct the problem, the engine must be disassembled and the problem should be taken care of. If all the cylinders have low compression, this may be the cause of poor performance of the engine. The excessive carbon in
the cylinders may be the cause of pre-ignition. The compression gauge should be used to ascertain this.
A compression gauge is used to determine how well a cylinder is sealed, when the valves are closed, and how well the air is compressed in the cylinder. Whenever a problem indicates that either of these might be affected and causing the problem, this test should be conducted. Compression losses normally affect the engine’s performance at all speeds. If it is definite that the engine’s problem exists at only certain speeds, then there is no need to conduct a compression test.
