Compression check
1. The results of compression measurements will help determine the technical condition of engine parts (pistons, rings, valves, cylinder head gasket). By analyzing the results, you can determine whether the loss of compression is caused by worn piston rings, valves and their seats, or the cylinder head gasket.
Note: To measure compression, the engine must be warmed up to operating temperature. The battery must be fully charged.
2. Clean the areas around the spark plugs before removing them (Use compressed air if possible, but a small brush or even a bicycle pump will also work). These measures are aimed at preventing dirt from entering the cylinders when measuring compression.
3. Remove the coils and spark plugs from the engine (see _ chapter 1 and 5).
4. Remove the fuel pump relay to de-energize its circuit (the relay is located in the fuse/relay box located in the engine compartment - see chapter 4, illustration 3.3).
5. Connect a compression gauge to the hole of the first spark plug (see illustration).
3.5. A compression gauge with an adapter screwed into the spark plug hole is preferable to a compression gauge that must be pressed during measurement; When taking measurements, the throttle must be in the fully open position
6. Rotate the engine crankshaft so that the pistons go through the compression stroke at least four times. At the same time, observe the readings of the compression gauge. Compression should increase quickly if the engine is healthy. Low compression after the first stroke of the piston, which rises during subsequent strokes, indicates wear on the piston rings. Low compression during the first and subsequent strokes of the piston indicates a loose valve fit or that the gasket under the cylinder head is leaking (A crack in the head could also be the cause). The presence of carbon deposits on the back of the valve heads can also cause reduced compression. Record the maximum compression reading.
7. Take measurements on the remaining cylinders and compare their results with the value given in the specifications.
8. Pour clean engine oil through the spark plug holes (approximately three pumps from a grease gun) and repeat the compression test.
9. If adding oil temporarily increases compression, then the reason for its decrease is wear on the piston rings. If there is no increase in compression, then the valves may be loose or burnt out, or air may leak through the head gasket. Leaks in closed valves can be a result of burnout of their seats, or may also occur due to damage to the surface of the disc or deformation of the stem.
10. Low compression in two adjacent cylinders is possible due to air leakage through the head gasket area located between the cylinders. This will also be indicated by the presence of coolant in the engine oil.
11. If the compression in one of the cylinders is slightly below the set limit, and slight malfunctions are noted when the engine is idling, the cause of this problem may be wear on the camshaft cam.
12. If the compression gauge readings are unusually high, the cause may be carbon deposits that have accumulated on the walls of the combustion chamber. If this assumption is confirmed, the head should be removed (And) cylinder block and clean off carbon deposits.
13. If the compression is below the set value, or the results of its measurement in the cylinders differ significantly from each other, it is advisable to carry out diagnostics by measuring the degree of vacuum. To do this, contact a service and repair station. After such a diagnosis, the sources and degree of depressurization of the engine cylinders will be accurately determined.
Diagnostics with measurement of the degree of vacuum
14. Carrying out diagnostics using a vacuum gauge allows you to get an idea of the processes occurring in the engine without much expense. During the test, the degree of wear of the piston rings and cylinder walls, cylinder head gaskets and intake manifold is determined, as well as compliance with the standard air-fuel mixture ratio, exhaust system patency, condition of the valves and their springs, correct adjustment of the ignition system and compliance with the standard valve timing.
15. Unfortunately, measuring the degree of vacuum alone does not provide an accurate diagnosis of the malfunction, so this procedure should be carried out in conjunction with other diagnostic tests.
16. To accurately interpret test results, it is important to consider the absolute values and rate of change of the vacuum gauge readings. Most vacuum gauges give readings in millimeters of mercury (mm. rt. Art.). The description of the procedure assumes that measurements are taken at sea level. As the atmospheric pressure increases or decreases, the vacuum gauge readings decrease. For every 300 m above 600 m above sea level, the vacuum gauge readings increase by approximately 25 mm. rt. Art.
17. Measure the degree of direct vacuum in the intake manifold, not transferred (in the throttle block) (see illustration). Make sure that all hoses are connected when taking measurements, otherwise incorrect readings will be obtained.
3.17. Using a simple vacuum gauge, you can make an accurate diagnosis; Before deciding on the need for major repairs, you should definitely perform this test
18. Before measuring the degree of vacuum, pre-warm the engine. Support the wheels and apply the parking brake. With the gear shift lever in position «PARK», start the engine and leave it idling.
Warning: Do not put your hands or vacuum gauge into the fan blades.
19. Take readings from the vacuum gauge; a normal working engine should create a stable vacuum of approximately 432 - 559 mm. rt. st (see illustration). The measurement results below indicate the following engine conditions.
3.19. Typical indicators of a vacuum gauge
- A) A persistent low reading on the vacuum gauge usually indicates a leaking gasket between the intake manifold and throttle body, a failed vacuum hose, extremely late ignition, or improper valve timing. Check the ignition timing using a stroboscope, and before removing the timing chain cover to check the position of the timing marks, eliminate other possible causes by performing the checks described in this chapter.
- b) If the vacuum gauge readings are unstable and 80-200 mm below the required value, it should be assumed that the intake manifold gasket on the intake duct is leaking or the fuel injector is failing.
- With) If the stable readings of the vacuum gauge are approximately 50-100 mm below the given value, it should be assumed that the valves are leaking in the closed position. Perform a compression test and a leak test to confirm your assumption.
- d) An unstable or abrupt decrease in pressure may be the result of a sticking valve or misfire. Perform a compression test and a leak test, and inspect the spark plugs.
- e) Rapid vibrations of the vacuum gauge needle with an amplitude of approximately 100 mm. rt. Art. idling of the engine in combination with increased smoke emission from the exhaust pipe indicates wear of the valve guides. Perform a leak test to confirm your assumption. If the needle begins to vibrate quickly as the engine speed increases, check the intake manifold gasket or cylinder head gasket for leaks and make sure. that the valve springs are not weakened, the valves are not burned out, and the ignition system operates without failure.
- f) Slight oscillation of the needle with an amplitude of approximately 100 mm. rt. Art. most likely indicates a problem in the ignition system. Carry out all relevant checks as during maintenance and, if necessary, check the ignition operation using a special analyzer.
- g) If the vacuum gauge needle fluctuates significantly, perform a cylinder compression test and a leak test to identify a failed engine cylinder or a leaking cylinder head gasket.
- h) If the needle oscillates slowly over a wide range, check the patency of the crankcase ventilation system (PCV), compliance with the idle air-fuel mixture ratio, as well as the tightness of the throttle block and intake manifold gasket.
- j) Raise the crankshaft speed to 2500 rpm and sharply release the gas pedal. If the engine is in normal operating condition, then the degree of vacuum should tend to zero and then exceed the normal value by approximately 130 mm. rt. Art. and return to its normal engine idle position. If the degree of vacuum slowly increases without reaching its maximum value when the throttle block is closed, the piston rings may be critically worn. If the arrow moves with a delay, then the passage of the exhaust system may be limited (The muffler or catalytic converter is often clogged). A simple way to identify a blocked element is to disconnect the exhaust system in the area in front of the element being tested and re-check the degree of vacuum.