The results of the compression test in the engine cylinders make it possible to judge the condition of a group of parts in the upper part of the engine (pistons, piston rings, valves and cylinder head gaskets). Namely, a decrease in compression may be due to leaks in the combustion chambers due to wear of the piston rings, damage to the plates and valve seats, burnout of the cylinder head gasket.
Check in the following order:
- clean the areas near the spark plugs by blowing with compressed air (in the absence of a compressor, blow the areas with an automobile, or even bicycle, pump). This is necessary in order to prevent dirt from entering the cylinders when measuring compression;
- remove all spark plugs;
- fully open the throttle;
- disable the ignition system by disconnecting the electrical connectors from the ignition coils. Also disconnect an electric socket of supply of tension to the fuel pump;
- insert the compression gauge into the spark plug hole;
- turn on the starter and rotate the crankshaft for at least seven compression strokes, watching the compression gauge readings. On a healthy engine, the pressure should build up quickly. Low pressure after the first piston stroke and a slow build-up on subsequent compression strokes indicates worn piston rings. If the pressure is low after the first piston stroke 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 reverse sides of the valve heads. Write down the highest compression value;
- repeat the measurement procedure for the remaining cylinders and compare the results with the technical data.
If the compression in the cylinders is below the minimum allowable value, inject a little engine oil into each cylinder through the spark plug hole, then recheck. If, after the introduction of oil, the compression increased, then we can make an unambiguous conclusion 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. Leakage through valves can be caused by burnt valve seats and/or valve lips, or by deformation or cracking of valve stems.
If the compression is equally low in only 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 the oil pan.
If the compression value in one of the cylinders is lower than in the others and the engine is unstable at idle, then the cause may be wear on the camshaft cams.
If the compression value exceeds the nominal value, then the combustion chamber is covered with carbon deposits. In this case, the cylinder head must be removed and carbon deposits removed.
If the compression in all cylinders is low, or very different for different cylinders, then it is necessary to test the combustion chambers for tightness, for which you need to contact a specialized workshop. As a result of the tests, leak locations must be accurately determined and a quantitative characteristic of the leak given.
After checking the compression, release the throttle and repair the ignition and fuel systems.