The engine must be warmed up to normal operating temperature and the battery 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. Release the pressure in the fuel system (see Chapter Power and exhaust systems). De-energize the power supply system by removing the fuel pump relay from the mounting block located in the engine compartment.
On diesel engines, release the latch and disconnect the electrical wiring from the fuel injection pump control unit to avoid fuel splashing and eliminate the risk of the engine starting spontaneously.
3. Remove all spark plugs/glow plugs from the engine (see Chapter Routine maintenance). Fix the throttle valve in the fully open position.
4. On models equipped with an ignition distributor, disconnect the central element from the distributor cover (coil wire) and ground it to the engine block. On models without a distributor, disconnect the wiring from each of the ignition coils.
5. Install the compression tester into the spark plug hole of the first cylinder - it is preferable to use a threaded attachment rather than a clamp-type attachment to connect the compression tester. Do not forget to open the throttle valve all the way during the test.
A — Checking the compression pressure in the third cylinder of a gasoline engine
B — Checking the compression pressure in the first cylinder of a diesel engine
A.
B.
In diesel engines, the compression ratio is much higher than in petrol engines, so to check the compression pressure, you will need a special compression tester with a measurement range of at least 40 bar. The compression tester with its adapter is screwed into the glow plug hole, or installed instead of the fuel injection nozzle. Due to the design features of the injectors used in the diesel engines discussed in this Manual, the compression tester should be screwed into the spark plug holes.
6. 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 on subsequent ones, indicates wear of the piston rings. If the pressure does not increase with further cranking of the shaft, this can be regarded as evidence of valve leaks or a breach of the cylinder head gasket (the possibility of cracks in the head is also possible). Deposits on the valve head faces can also cause a decrease in compression pressure. Record the maximum compression tester reading.
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 into each cylinder through the spark plug hole (two to three teaspoons) and check again.
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.
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.
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 cams of the corresponding valve drive on the camshaft should be assessed.
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.
If the measurement results are excessively low, or if the pressure distribution between the cylinders is uneven, it will be useful to test the engine for leaks in a car service workshop. This test will allow you to accurately determine the source of the leak and assess its severity.
Checking the block for leaks
During this test, the speed of the compressed air pumped into the cylinders and the leak locations are determined. This test is an alternative to the compression test. Moreover, from many points of view, it is much more effective, since it is easier to visually identify the source of the leak than to understand the results of the compression measurement.
The equipment needed to check for leaks is not generally available, so this test will have to be entrusted to service center specialists.
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