Vacuum gauge

Why Vacuum or performance gauges are useful for seeing the condition of your engine.

Vacuum or performance gauges give information about the condition of the engine.They are calibrated in mercury inches (in. Hg), usually from 0 to 30, and record the difference in pressure between the outside atmosphere and the inlet manifold.

Performance gauges also have coloured sectors to mark different engine conditions. When an engine is in good condition, at idling speed the indicator needle should be steady in the 17-21 in. Hg range (a green sector).At driving speed, the needle should be in the 10-18 in. Hg range—the higher the better (a blue sector).

Steady, but low, vacuum readings can indicate retarded ignition or weak compression on cylinders.  Fluctuating vacuum readings indicate an ignition fault or lack of compression on one or more (but not all) cylinders. A very low vacuum reading usually indicates a leaking inlet manifold.

Some possible fault using a vacuum gauge

Steady reading below 5 in. Hg shows leaking manifold or carburettor gasket or possible injection air leak.

Weak valve springs cause the needle to swing back and forth erratically when you press the accelerator. A fast, vibrating reading between 14 and 19 points to worn valve guides.

Between 8 and 14 in. Hg is an indication of incorrect valve timing . A reading dropping 3-5 in. Hg from normal indicates a sticking valve or worn contact breaker in older cars.

When the needle drifts between 5 and 19 in. Hg the cause may be a compression leak between two or more cylinders.If reading drops to zero and re-turns to 22 while revving, piston rings may be worn.

A reading that is consistently higher than normal (17 to 21 in. Hg) indicates a blocked air cleaner or carburettor air fault.