The engine must be warmed up to normal operating temperature and the battery fully charged.
1. Start by cleaning the candle niches (blow them with compressed air, the source of which in this case is perfect for an ordinary bicycle pump). The main objective is to prevent debris from entering the cylinders during measurements.
2. Relieve pressure in the supply system (see chapter Power and exhaust systems). De-energize the power supply system by removing the fuel pump relay from the mounting block installed in the engine compartment.
On diesel engines, release the latch and disconnect the wiring from the injection pump control unit, which will avoid fuel splashing and eliminate the risk of spontaneous engine starting.
3. Remove all spark/glow plugs from the engine (see chapter Current service). Lock the throttle in the fully open position.
4. On models equipped with an ignition distributor, disconnect the central (coil wire) and ground it to the engine block. On models without a distributor, disconnect the electrical wiring from each of the ignition coils.
5. Install the compression tester in the spark plug hole of the first cylinder - it is preferable to use a threaded nozzle rather than a clamping type to connect the compression tester. Do not forget to open the throttle valve all the way during the check.
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 gasoline engines, so a special compression gauge with a measuring range of at least 40 bar is required to check the compression pressure. The compression tester is screwed into the glow plug hole with its adapter nozzle, 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 gauge should be screwed into the spark plug holes.
6. Turn the engine over at least seven cycles and read the gauge. On a healthy engine, the pressure should build up fairly quickly. Low pressure on the first stroke, accompanied by a gradual increase in subsequent strokes, indicates wear on the piston rings. If the pressure does not rise with further rotation of the shaft, this can be regarded as evidence of valve leaks or a leak in the cylinder head gasket (the possibility of cracks in the head is also not excluded). The accumulation of deposits on the working facets of the valve plates can also lead to a decrease in compression pressure. Record the maximum pressure gauge reading.
7. Repeat the procedure for the remaining engine cylinders. Compare results with regulatory requirements (see Specifications).
8. If the measurement is too low, pour some engine oil into each of the cylinders through the spark plug hole (two to three teaspoons) and recheck.
If the addition of oil leads to an increase in pressure, this indicates worn piston rings. If the compression does not increase, the cause of the decrease is valve leaks or a leaking head gasket. Valve leaks can be associated with burn-out of their seats, or deformation / burn-out 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 jumper between these cylinders. The presence of coolant in the engine oil will confirm this assumption.
If the pressure in one of the cylinders is about 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 corresponding valve drive cams 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 in order to carry out decarbonization.
In case of excessively low measurement results, as well as in case of a violation of the uniform distribution of pressure between the cylinders, it will be useful to test the engine for leaks in a car service workshop. This check will pinpoint the source of the leak and assess its severity.
Checking the unit for leaks
During this test, the rate of exit from the cylinders of the compressed air pumped into them and the leaks directly 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 a leak than to comprehend the results of a compression measurement.
The equipment needed to test for leaks is not publicly available, so this test will have to be entrusted to the service center specialists.
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