A vacuum gauge monitors the engine's vacuum. A lot can be diagnosed by simply reading a vacuum gauge. The proper tune-up, fuel settings and spark timing can all be adjusted and fine tuned by watching and understanding the vacuum gauge. The vacuum gauge shows the amount of pressure within the engine's intake manifold.
Simply put, an engine is nothing more than a giant air pump. The pistons move up and down within the cylinder walls and create an air pumping machine. As the pistons move downward on the engine's intake stroke, they pull air through the fuel system and intake it into the engine. The vacuum gauge shows an air loss or leak as a loss of vacuum. Where the loss comes in relation to the pistons' travel defines what the problem might be.
While idling, a well-tuned engine will show a steady vacuum reading on the gauge. Quickly stepping on the accelerator pedal and letting off again will cause the vacuum level to drop and then return to a steady reading. The symptoms of a poor ignition system will show up as a slight variation of the vacuum pressure on the vacuum gauge. The gauge's needle will move back and forth on the dial, and the reading will vary 1 or 2 inches (2.5 to 5 cm) on the dial's scale.
An unsteady vacuum reading at idle is typically a sign of bad spark plugs, plug wires or even a bad coil. All of these parts may appear to be operating as they should, but the vacuum gauge will show that something is not up to par. Testing or replacing these items and monitoring any change on the vacuum dial's reading will show the defective part as well as its fix.
The vacuum reading fluctuating 3 to 5 inches (7.5 to 12.5 cm) while idling is a sure sign that the intake or exhaust valves are not opening and closing properly. This could be due to a worn valve guide or valve stem, a weak valve spring or even a bad lifter or cam lobe. Sticking valves rob an engine of power and could cause the engine to fail or to burn a piston.
A vacuum reading that drops rapidly then returns in a rhythmic manner is often the sign of a blown head gasket or a cracked head. The failure of the vacuum gauge to rise steadily as the engine is revved up is a sign of excessive exhaust back pressure or of an exhaust blockage. Blown gaskets and intake problems are depicted by a vacuum reading much lower than normal at idle, but the reading will remain steady.