Vacuum gauges are among the mechanic's most useful but lesser known tools. Intake manifold vacuum has long been known as an indicator of load on the engine; more throttle and more load equals less vacuum, and vice versa for more vacuum. A fluctuating vacuum reading can mean a number of things, many of which will cause a noticeable power loss or misfire.
Slow, Wide Fluctuation
A needle that fluctuates fairly slowly and over a wide range usually indicates a problem with either the carburetor adjustment or the engine's air and fuel management system. As vacuum rises, it sucks more fuel through the carburetor, throwing off the air-to-fuel ratio and causing a drop in manifold vacuum. The same thing might happen if you've got a bad throttle position, manifold air pressure or mass airflow sensor.
Quick, Short Fluctuations
A vacuum reading that very quickly drops and rises a couple inches of mercury or psi generally indicates some sort of ignition system malfunction. You might have a misfire as the result of a bad coil or crossfiring in the distributor, or you may have a spark plug with too small or large a gap. If the needle is truly spastic, you could have a multicylinder ignition misfire. On a computer-controlled car, a bad cam or crank position sensor could cause this, but these will manifest in other and more noticeable ways.
Wide Fluctuation at Idle
A wide fluctuation at idle generally means that something's gone awry between two of the engine's cylinders; one is pulling when it should be pushing, the other may be doing nothing. The only way this is going to happen is if you've either blown a head gasket or cranked the head, in which case you'll see other symptoms. You're likely to see excessive exhaust smoke that smells like fuel, antifreeze, oil or all three.
Low and periodically fluctuating vacuum readings under all conditions is a classic indicator of worn piston rings or an engine in need of a rebuild. The worn rings or excessively large bore will allow combustion gases to escape into the crankcase and won't seal well enough to pull the standard vacuum. Additional symptoms include excessive crankcase pressure as a result of ring blow-by; you'll see this as a hot wind puffing out of your valve cover oil filler hole.
A vacuum gauge needle that vibrates rapidly at idle and then seems to smooth out with rpm can indicate bad valve guides or a bad cam lobe. The bad guides allow the valve to move around a bit while the valves are at their lowest opening point, and the bad lobe will effectively kill that cylinder at low lift. Intermittent drops in vacuum at idle and under cruise could indicate a sticking valve, and needle drop of vibration under acceleration can indicate a bad valve or valveseat. Hydraulic lifter bleed-off will manifest as an intermittent vacuum drop at idle, and incorrect valve lash will mimic the effects of a bad valve.
Engine idle isn't a steady thing; it actually fluctuates regularly as the pistons suck in regular gulps of air. This is especially true on large displacement engines with a relatively low cylinder count, like a 500 cubic inch V-8 or a 300 cubic inch six cylinder. Vacuum gauges have dampeners inside to keep the needle from constantly vibrating, and those dampeners do wear out over time. Additionally, aftermarket vacuum gauges, particularly those designed for diagnosis as opposed to installation, may require a separate dampener to keep from giving a false reading under normal conditions.