Note: The engine must be at normal operating temperature and the battery must be fully charged.
1. Start by cleaning the spark plug niches (blow them out with compressed air, a regular bicycle pump is a great source of air in this case). The main objective is to prevent debris from entering the cylinders during measurements.
2. Remove all spark plugs from the engine (see Chapter Routine maintenance).
3. Fix the throttle valve in the fully open position.
4. Immobilize the power supply system by removing the PGM-FI main relay fuse (see Chapter Power and exhaust systems).
5. Also disable the ignition system by disconnecting the connectors of the primary distributor circuit (see Chapter Engine electrical equipment).
6. Install the compression gauge into the spark plug hole of the first cylinder (see accompanying illustration).
7. Crank the engine for at least seven strokes and read the meter reading. On a serviceable engine, the pressure should increase fairly quickly. Low pressure on the first stroke, accompanied by a gradual increase in the subsequent ones, indicates wear of the piston rings. If the pressure does not increase with further cranking, this can be considered evidence of valve leaks or a leak in the cylinder head gasket (the possibility of cracks in the head is also possible). Record the maximum reading on the compression gauge.
8. Repeat the procedure for the remaining engine cylinders. Compare the results with the standard requirements (see Specifications).
9. If the measurement results are excessively low, pour some engine oil into each cylinder through the spark plug hole (two to three teaspoons) and check again.
10. If adding oil increases the pressure, this indicates wear of the piston rings. If the compression does not increase, the reason for its decrease is valve leaks or a leak in the head gasket. Valve leaks may be associated with burnout of their seats, or deformation/burnout or mechanical damage to the working chamfers of the plates.
11. If the compression pressure is equally low in two adjacent cylinders, then with a high degree of probability we can talk about a violation of the integrity of the head gasket within its bridge between these cylinders. The presence of coolant in the engine oil will confirm this assumption.
12. If the pressure in one of the cylinders is approximately 20% lower than in the others and this is accompanied by a violation of the stability of the idle speed, the degree of wear of the exhaust valve drive cam on the camshaft should be assessed.
13. An unusual increase in compression pressure is usually a consequence of carbon formation in the combustion chambers. In this case, the cylinder head must be removed for decarbonization.
14. In case of excessively low compression pressure, as well as in case of severe violation of uniformity of its distribution between the cylinders, it will be useful to test the engine in the conditions of a car service workshop. Such a check will allow to accurately determine the location of the pressure leak and assess the degree of its severity.
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