To obtain accurate test results, the engine must be warmed up to normal temperature and the battery must be fully charged.
1. Remove the spark plugs.
2. Open the throttle fully.
3. Turn off the ignition by connecting the coil wire to ground. On injection engines, turn off the fuel pump (see subsection 5.1.4).
4. Insert the compression gauge into the spark plug hole of cylinder 1.
5. Turn on the starter and watch the pressure gauge. On a working engine, the pressure should build up quickly. Low pressure after the first stroke of the piston and a slow build-up during subsequent compression strokes indicates worn piston rings. If the pressure is low after the first stroke of the piston and does not build up during subsequent compression strokes, the cause is a leak in the valves or a leaky gasket or cylinder head. Record the highest compression reading.
6. Repeat the procedure for the remaining cylinders.
7. Inject some oil (10–15 cm³) into the cylinders. If the compression increases after injecting oil into the cylinder, then we can conclude that the piston rings are worn out. If the compression increases slightly, then the leak is through the valves or the head gasket.
8. If the compression is equally low only in two adjacent cylinders, then the most likely cause is a burnt gasket between these cylinders.
9. If the compression value exceeds the norm, the combustion chamber is covered with carbon deposits. In this case, the cylinder head must be removed and the carbon deposits removed.
10. If there is a significant difference in compression in the cylinders, the car should be taken to a service station to have the cylinder head and block checked for leaks using the pressure testing method.