The results of the compression test in the engine cylinders allow us to judge the condition of a group of parts in the upper part of the engine (pistons, piston rings, valves and cylinder head gaskets). Specifically, a decrease in compression may be due to a lack of tightness in the combustion chambers due to wear of the piston rings, damage to the valve plates and seats, or burnt-out cylinder head gaskets.
Perform the check in the following order:
- clean the areas near the spark plugs by blowing them with compressed air (if you don't have a compressor, blow out the areas with a car or even a bicycle pump). This is necessary to prevent dirt from getting into the cylinders when measuring compression;
- remove all spark plugs;
- open the throttle fully;
- disable the ignition system by disconnecting the electrical connectors from the ignition coils. Also disconnect the electrical connector that supplies voltage to the fuel pump;
- insert the compression tester into the spark plug hole;
- turn on the starter and turn the crankshaft through at least seven compression strokes, monitoring the compression gauge readings. On a properly functioning engine, the pressure should build up quickly. Low pressure after the first piston stroke and a slow increase during subsequent compression strokes indicates wear of the piston rings. If the pressure is low after the first stroke of the piston and does not increase during subsequent compression strokes, then the cause is a leak in the valves or a leak in the cylinder head gasket (the cause may also be the formation of cracks in the head). A decrease in compression can also be caused by carbon deposits on the backs of the valve heads. Record the highest compression reading;
- repeat the measurement procedure for the remaining cylinders and compare the results with the technical data.
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If the compression in the cylinders is below the minimum permissible value, inject a small amount of engine oil into each cylinder through the spark plug hole, then repeat the check. If the compression increases after adding oil, then we can definitely conclude that the piston rings are worn out. If the compression has increased slightly, then the leak is through the valves or the cylinder head gasket. Valve leakage can be caused by burnout of the valve seats and/or working edges, as well as deformation of the valve stems or the formation of cracks on them.
If the compression is equally low only in two adjacent cylinders, then the most likely cause is a burnt gasket between these cylinders. As confirmation, traces of coolant may appear in the combustion chambers or oil pan.
If the compression value in one of the cylinders is lower than in the others and the engine runs unsteadily at idle speed, then the cause may be wear of the camshaft cams.
If the compression value exceeds the nominal value, it means that the combustion chamber is covered with carbon deposits. In this case, the cylinder head must be removed and the carbon deposits removed.
If the compression in all cylinders is low, or varies greatly between cylinders, then it is necessary to test the combustion chambers for leaks, for which you should contact a specialized workshop. As a result of the tests, the leak locations must be accurately identified and a quantitative characteristic of the leak must be given.
After checking the compression, release the throttle valve and restore the ignition and fuel systems.