Variants of vacuum gauge readings when diagnosing engine malfunctions
Connect a vacuum gauge to the intake manifold. Start and warm up the engine to normal operating temperature.
Read the vacuum gauge readings.
- If the engine is in normal condition, the readings of the vacuum gauge should be constant and be 430÷560 mmHg Art.
- Low continuous readings vacuum gauge indicate damage to the gasket between the intake manifold and throttle body, damage to the vacuum hose, incorrectly set ignition timing or incorrectly set valve timing.
- If the vacuum gauge reading below the norm by 80÷200 mm Hg. Art. and hesitate, then the inlet pipe gasket near the inlet may be damaged or the injector may be defective.
- If the testimony constantly fall by 50÷100 mm Hg. Art. relatively stable value, the valves may be damaged. In this case, the compression in the engine cylinders should be measured (see Section Compression pressure test).
- If the testimony fall, but not regularly, then the valve may be stuck or there is a malfunction in the ignition.
- If at constant idle speed the readings fluctuate rapidly with an amplitude of about 100 mm Hg. Art., and smoke is coming out of the exhaust pipe, the valve guides may be damaged.
- If the testimony fluctuate rapidly as idle speed increases, then the intake manifold gasket or cylinder head gasket may be damaged, valve springs may be weak, valves may be burned, or the ignition may be misfiring.
- Small fluctuations of about 25 mm Hg. Art. usually associated with failures in the ignition system.
- If the testimony fluctuate greatly, then the cylinder head gasket or cylinder may be damaged.
- If the arrow moves slowly over 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 pipeline is damaged.
Check how quickly the readings of the vacuum gauge are restored after a sharp and full opening of the throttle valve and its return to its original position. If the engine is in normal condition, the reading drops to near zero, then rises by about 130 mmHg. 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.