Note: The engine must be at normal operating temperature and the battery must be fully charged.
1. Start by cleaning out the spark plug wells (blow them out with compressed air, a regular bicycle pump works great in this case). The main goal is to prevent debris from getting into the cylinders while you're taking measurements.
2. Remove all spark plugs from the engine (see Chapter Settings and ongoing maintenance).
3. Fix the throttle valve in the fully open position.
4. Disconnect the ignition coil high voltage wire from its terminal on the distributor cap and ground it to the block ground (use a jumper wire with alligator clips). You should also de-energize the fuel pump circuit (see Chapter Power and exhaust systems).
5. Install the compression gauge into the spark plug hole of the first cylinder.
6. Crank the engine for at least seven strokes and read the meter reading. On a serviceable engine, the pressure should build up fairly quickly. Low pressure on the first stroke, followed by a gradual increase on subsequent ones, indicates wear of the piston rings. If the pressure does not rise with further cranking of the shaft, this can be regarded as evidence of valve leaks or a leak in the cylinder head gasket (there is also a possibility of cracks in the head). The accumulation of deposits on the working chamfers of the valve plates can also lead to a decrease in compression pressure. Record the maximum reading of the compression meter.
7. Repeat the procedure for the remaining engine cylinders. Compare the results with the standard requirements (see Specifications).
8. If the measurement results are excessively low, pour a little engine oil (two to three teaspoons) into each cylinder through the spark plug hole and repeat the check.
9. 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.
10. If the compression pressure is equally low in two adjacent cylinders, then with a high degree of probability we can speak 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.
11. 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 cam of the corresponding exhaust valve drive on the camshaft should be assessed.
12. An unusual increase in compression pressure is usually the result of excessive carbon formation in the combustion chambers. In this case, the cylinder head must be removed for decarbonization.
13. If the measurement results are excessively low, or if the pressure distribution between the cylinders is not uniform, it will be useful to test the engine for leaks in a car service workshop. Such a test will allow you to accurately determine the source of the leak and assess its severity.