Compression values in the engine cylinders allow you to judge the condition of a group of parts in the upper part of the engine (pistons, piston rings, valves and cylinder head gasket). 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.
Start and warm up the engine to normal operating temperature, then stop it and wait 10 minutes for the temperature of the catalytic converter to decrease.
Clean areas near spark plugs by blowing with compressed air (in the absence of a compressor, blow the areas with a car or even a bicycle pump). This is necessary in order to prevent dirt from entering the cylinders when measuring compression.
Remove all spark plugs.
Disable the ignition system by disconnecting the ignition coil primary from the ignition distributor.
Fully open the throttle.
Insert the compression gauge into the spark plug well.
Pic. 2.24. Using a compression gauge to measure compression in engine cylinders
Engage the starter and rotate the crankshaft for at least seven compression strokes, monitor the compression gauge (pic. 2.24). 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 indicate 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 may be caused by carbon deposits on the reverse sides of the valve heads. Write down the highest compression value.
Measure the compression in the remaining cylinders and compare the results with the technical data given at the beginning of the section.
If the compression is below the minimum 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, the leak is through the valves or the cylinder head gasket. Valve leakage can be caused by burnt valve seats and/or valve lips, deformed or cracked valve stems.
If the compression is equally low in only two adjacent cylinders, then the most likely cause is a burnt gasket between them. As confirmation, traces of coolant may appear in the combustion chambers or the oil pan.
If the compression in one of the cylinders is lower than in the others, and the engine is unstable at idle speed, then the cause may be wear on the camshaft cam.
If the compression exceeds the nominal value, then there is carbon deposits on the combustion chamber. 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 check the combustion chambers for leaks, for which contact a specialized car workshop. As a result of the tests, the leak locations must be accurately determined and the quantitative characteristic of the leak given.
After checking the compression, release the throttle and restore the ignition system.