Basic positions and fluctuations of the vacuum gauge needle when diagnosing engine faults
Connect a vacuum gauge to the intake manifold. Start and warm up the engine to normal operating temperature.
Read the vacuum gauge. If the engine is in normal condition, the vacuum gauge readings should be 430-560 mm Hg, and the vacuum gauge needle should be in place.
Low constant readings vacuum gauge indicate that the gasket between the intake manifold and the throttle body is damaged, the vacuum hose is damaged, the ignition timing is incorrectly set, or the valve timing is incorrectly set.
If the vacuum gauge readings below normal 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, it is necessary to measure the compression in the engine cylinders.
If the readings are falling, but not regularly, then perhaps the valve is jammed or there is an ignition failure.
If the readings fluctuate rapidly with an amplitude of about 100 mm Hg., at constant idle speed, and smoke 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, 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.
Check how quickly the vacuum gauge readings recover after the throttle valve is opened sharply and fully and 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 again decrease 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.