Caution: The engine must be at normal operating temperature and the battery must be fully charged for this test.
Start by cleaning the area around the spark plugs before you remove them to prevent dirt from getting into the cylinders when doing a compression test. If you have a source of compressed air, then you should use it for this purpose, a soft brush or even an ordinary bicycle pump may also work.
Remove all spark plugs from the engine.
Lock the throttle valve fully open.
Disconnect the ignition coil wire.
Insert the tip of the compression tester into the opening of the first spark plug.
Compressometer
Crank the engine with the starter for at least seven compression strokes while watching the gauge. Pressure should build up quickly in a healthy engine. Low pressure on the first stroke, accompanied by a gradual increase in pressure on subsequent strokes, indicates worn piston rings. Low pressure on the first strokes that does not increase on subsequent strokes may be due to valve leaks or a damaged cylinder head gasket (Could also be a cracked head). The cause of low pressure may be deposits on the valve heads. Write down the obtained pressure values.
Repeat this operation for the rest of the cylinders and compare the measurement results with the technical data.
Add a little engine oil to each cylinder through the spark plug hole and repeat the measurements.
If the pressure has increased after adding oil, the piston rings are definitely worn. If the pressure has not increased significantly, the leak is in the valves or the cylinder head gasket. Leakage through valves can be caused by burnt valve seats and/or valve faces, or cracked, deformed, or bent valves.
If pressure is low in two adjacent cylinders, there is a good chance that the cylinder head gasket between them is damaged. The appearance of coolant in the combustion chambers or the crankcase is evidence in favor of this assumption.
If the pressure in one cylinder is about 20% lower than in the others, and the engine feels uneven at idle, then a damaged exhaust cam on the camshaft may be the cause.
If the pressure is unusually high, then the combustion chambers may be covered with carbon deposits. If so, the cylinder head needs to be removed and cleaned.