Read the instrument readings. A normal engine should produce a vacuum of 430 to 560 mmHg. Art. The following signs may help assess the condition of the engine:
- A low value indicates a leaking gasket between the intake manifold and throttle body, a leaky vacuum hose, «later» ignition or failure of the gas distributor phases. Check the ignition setting and other possible causes.
- If the value is below normal by 75-200 mm Hg. Art. and changes towards low values, this indicates a leak in the intake manifold gasket.
- If the arrow periodically falls at a constant rate of 50-100 mm Hg. Art., then there may be a leak in the valves. Check compression.
- Irregular drop or flickering of the needle can be caused by cap sticking or misfiring. Check compression and spark plugs.
- Rapid vibration within 100mmHg Art. combined with exhaust smoke indicates worn valve guides. Check for leaks in the gaskets, the condition of the valve springs and the ignition setting.
- Small changes within 25 mm Hg. Art. indicate problems in the ignition system.
- If fluctuations are significant, then check the compression or the presence of leaks in the cylinders or gaskets.
- If the needle moves slowly over a wide range, then check for a clogged crankcase ventilation system, correct fuel-air ratio, leaks in the throttle body or gasket.
- Check the return of the gauge pointer after a quick opening of the throttle from zero (with an increase in speed up to 2500 rpm) to the desired value. If the vacuum releases slowly, the piston rings may be worn out or there may be a problem with the exhaust system.