Read the instrument readings. A normal engine should produce a vacuum of 430 to 560 mm Hg. The following signs can help assess the condition of the engine:
- A low reading indicates a leaking intake manifold to throttle body gasket, a leaking vacuum hose, retarded ignition timing, or poor valve timing. Check the ignition timing and other possible causes.
- If the value is 75-200 mmHg below normal and changes towards lower values, this indicates a leak in the intake manifold gasket.
- If the arrow periodically drops at a constant rate of 50-100 mm Hg, then there may be a leak in the valves. Check the compression.
- An irregular drop or jitter in the needle may be caused by a stuck cap or misfiring. Check compression and spark plugs.
- A rapid vibration of around 100 mmHg combined with smoke from the exhaust pipe indicates worn valve guides. Check for leaks in the gaskets, condition of the valve springs and ignition timing.
- Small changes within 25 mmHg indicate problems in the ignition system.
- If the fluctuations are significant, check the compression or check for leaks in the cylinders or gaskets.
- If the arrow moves slowly over a wide range, check whether the crankcase forced ventilation system is clogged, whether the fuel-air ratio is correct, or whether there are leaks in the throttle body or in the gasket.
- Check the return of the vacuum gauge needle after quickly opening the throttle valve from zero (when increasing the speed to 2500 rpm) to the desired value. If the vacuum drops slowly, there may be worn piston rings or a problem with the exhaust system.