Note: When measuring compression, the engine must be fully warmed up and the battery must be fully charged. On engines with carburetors, make sure the air damper is fully open.
2. Clean the spark plug seat before removing them (you can use compressed air, small brushes, and even a bicycle pump). Prevent dirt from entering the cylinders.
3. Remove the spark plugs (Chapter 1).
4. Open the throttle valve fully.
5. Disconnect the central high-voltage wire from the distributor and ground it to ground. Ensure reliable grounding. On engines with injection, additionally disconnect the power supply to the injectors (see Chapter 4).
6. Install the compression gauge into the spark plug socket of the 1st cylinder.
7. Rotate the crankshaft at least 7 compression strokes and read the pressure gauge. On a "healthy" engine, the pressure rises quickly. Low pressure on the first stroke and a subsequent gradual decrease in pressure on subsequent strokes indicates wear of the piston rings. Low pressure on the first stroke without increasing on subsequent strokes indicates leaks through the valves or a breakthrough through the gasket (a less likely cause is a crack in the head). Coking on the outer part of the valve plate due to throttling of the flow can also cause low pressure. Record the upper measured pressure value.
8. Repeat steps 6 and 7 for the other cylinders and compare the results with the specifications.
9. Add some motor oil (about 3 cm³) into each cylinder and repeat the tests.
10. If the compression pressure increases after adding oil, this clearly indicates worn piston rings. If the pressure does not increase, leaks are coming from the valves or gasket. Valve leaks may be due to improper clearance adjustment, burnt valve seats and/or chamfers, cracks in the valves or their bending.
11. If two adjacent cylinders have equally low compression, there is a high probability of a head gasket failure between them. The appearance of coolant in these cylinders will confirm this version.
12. If one of the cylinders has a compression pressure approximately 20% lower than the others and the engine has a slightly rough idle, this may indicate a worn camshaft lobe.
13. If the compression pressure is unusually high, there are too many coke deposits in the combustion chamber. In this case, remove the cylinder head and clean the combustion chamber from carbon deposits.
14. If the compression pressure varies significantly from cylinder to cylinder, it would be a good idea to have a special test performed at a service station to accurately determine the causes of the leaks and how to fix them.