Vacuum gauge readings options for engine fault diagnostics
1. Using a vacuum gauge, you can get information about the condition of the engine and determine if the cylinder head gasket is blown, the fuel system, piston rings or valves are damaged, the fuel injection system is incorrectly adjusted, etc. Unfortunately, vacuum gauge readings can be misinterpreted, so these measurements should be used in combination with other engine testing methods. Both the absolute value of the vacuum gauge readings and the rate of change are important for a correct determination.
2. Connect a vacuum gauge to the intake manifold. Start and warm up the engine to normal operating temperature.
3. Read the vacuum gauge readings.
- If the engine is in in normal condition, the vacuum gauge readings should be constant and amount to 430÷560 mm Hg.
- Low constant readings vacuum gauge indicate damage to the gasket between the intake manifold and the throttle body, damage to the vacuum hose, incorrectly set ignition timing or incorrectly set valve timing.
- If the vacuum gauge readings below the norm by 80÷200 mm Hg and fluctuate, then the intake manifold gasket near the inlet may be damaged or the injector may be faulty.
- If the readings constantly fall by 50÷100 mm Hg relative to the stable value, then the valves may be damaged. In this case, you should measure the compression in the engine cylinders (see section Checking the compression pressure).
- If the readings they fall, but not regularly, then perhaps the valve is jammed or there is an ignition failure.
- If at constant idle speed the readings fluctuate rapidly with an amplitude of about 100 mm Hg., and smoke is coming from the exhaust pipe, the valve guides may be damaged.
- If the readings fluctuate rapidly as idle speed increases, then the intake manifold gasket or cylinder head gasket may be damaged, the valve springs may be weak, the valves may be burnt, or there may be an ignition failure.
- Small fluctuations of about 25 mm Hg. are usually associated with failures in the ignition system.
- If the readings fluctuate greatly, then the cylinder head gasket or cylinder may be damaged.
- If the arrow moves slowly within a wide range of values, then the crankcase ventilation system may be clogged, the intake manifold gasket or the gasket between the throttle body and the manifold may be damaged.
4. Check how quickly the vacuum gauge readings recover after the throttle valve is opened sharply and fully and then returned to its original position. If the engine is in normal condition, the readings will drop to almost zero, then increase by about 130 mm Hg above normal and then decrease again to the previous values at constant idle speed. If the readings recover slowly and pass through the maximum after closing the throttle valve, then the piston rings may be damaged. If there is a long delay, then the exhaust system may be clogged.
(Information taken from this resource: «BMWman.ru»)