Basic positions and fluctuations of the gauge needle when diagnosing engine malfunctions
Using a vacuum gauge, you can get information about the condition of the engine and determine that the cylinder head gasket is blown, the rings are damaged, the power system is incorrectly adjusted, the valves are damaged, etc. Unfortunately, the readings of the vacuum gauge can be misinterpreted, so these measurements should only be used in combined with other methods of checking the engine. For a correct determination, both the absolute value of the vacuum gauge readings and the rate of change are important.
Order of execution
1. Connect a vacuum gauge to the intake manifold. Start and warm up the engine to normal operating temperature.
2. Read the vacuum gauge readings. If the engine is in normal condition, the vacuum gauge reading should be 430–560 mm. rt. Art., and the needle of the vacuum gauge should be in place.
A low, constant reading on the vacuum gauge indicates that the gasket between the intake manifold and the throttle body is damaged, the vacuum hose is damaged, the ignition timing is incorrect, or the valve timing is incorrect.
If the readings of the vacuum gauge are below the norm by 80–200 mm. rt. Art. and fluctuate, the intake manifold gasket near the inlet may be damaged or the injector may be defective.
If the readings constantly fall by 50-100 mm. rt. Art. relatively stable value, the valves may be damaged. In this case, it is necessary to measure the compression in the engine cylinders.
If the readings drop, but not regularly, then the valve may be stuck or there is a malfunction in the ignition.
Small fluctuations of about 25 mm. rt. Art. usually associated with failures in the ignition system.
If the reading fluctuates greatly, then the cylinder head gasket or cylinder may be damaged.
If the needle moves slowly over a wide range of values, then the positive crankcase ventilation system may be clogged, the intake manifold gasket or the gasket between the throttle body and the manifold is damaged.
3. Check how quickly the readings of the vacuum gauge are restored after a sharp and full opening of the throttle valve and returning it to its original position. If the engine is in normal condition, the reading drops to almost zero, then rises by about 130 mm. rt. Art. above normal and again decrease to the previous values at constant idle speed. If the reading recovers slowly and goes through a maximum after closing the throttle, then the piston rings may be damaged. If there is a long delay, then the exhaust system may be clogged.
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