Note. To perform this test, the engine must be warmed up to normal operating temperature and the battery must be fully charged.
2. Start the parade by removing the spark plugs by cleaning the area where they are located (use compressed air whenever possible). This is necessary to prevent dirt from entering the cylinders when performing compression welding.
3. Disconnect the electrical connector and remove the ignition coils (see chapter 5). Then remove all spark plugs from the engine (see chapter 1).
4. Lock throttle in wide open position.
5. Shut off the fuel pump by tying the fuel pressure relief (see chapter 4).
6. Install a compression gauge in the spark plug hole. Use a compression gauge with a threaded connector that screws into the spark plug hole. Do not use a compression tester that must be pressed by hand to ensure tightness. When performing the test, open the throttle to the maximum (pic. 3.6).
Pic. 3.6. Use a compression gauge with a threaded connector that screws into the spark plug hole
7. Crank the engine through at least seven compression strokes and read the compression gauge. On a good engine, compression should build up quickly. Low compression on the first stroke and then a gradual increase in pressure on the remaining strokes is a sign of worn piston rings. A low reading on the first stroke that does not increase on the remaining strokes indicates valve leaks or a burnt cylinder head gasket (Cracks in the head could also be the cause). The cause of low compression may be deposits on the reverse sides of the valve heads. Record the highest compression gauge reading obtained.
8. Repeat the procedure for the remaining cylinders and compare the results with the values given in Specifications at the beginning of this chapter.
9. Add some engine oil (about a full teaspoon) to each cylinder (through the spark plug holes) and repeat the compression test.
10. If the addition of oil temporarily increases compression, this indicates pressure loss due to worn piston rings. The lack of significant improvement suggests that the cause may be valve leaks or a burnt cylinder head gasket. Leakage through valves can be caused by burnt valve seats and/or valve faces, as well as buckling, cracking or deformation of valves.
11. Equally low compression in two adjacent cylinders is almost certainly the result of a burn-out or breakdown of the cylinder head gasket between them. This can confirm the presence of coolant in the combustion chamber or crankcase.
12. If the pressure in one cylinder is slightly lower than the others and the engine runs slightly rough at idle, the cause may be worn camshaft lobes.
13. If the compression is abnormally high, there may be soot in the combustion chambers. In this case remove a head of cylinders and remove a deposit.
14. If the compression has fallen to the specified minimum value or varies significantly between cylinders, it is recommended to check for leaks, for which you should contact a service station. This weld will show exactly where the leak is and how serious it is.