Connecting the Gauge
There’s engine vacuum – and then there’s engine vacuum. When measuring engine vacuum for diagnostics, it’s important that you tap into the plenum chamber rather than on the throttle body itself. In some cases, throttle body vacuum connections will be of ‘ported vacuum’, which changes in an odd way as the throttle is opened. Also make sure that all the factory vacuum lines remain connected (that includes brake booster, charcoal canister, distributor vacuum advance [if fitted] MAP sensor, etc) before starting diagnostics, and the engine should also be up to normal operating temperature.
Normal Engine at Idle
When the engine is healthy and working correctly, at idle the needle of the gauge should be steady and within the range of 17 – 22 inches of mercury.
Normal Engine when Throttle Snapped Open and Closed
Vacuum should drop to 3 and then momentarily climb to 25 – 28 before settling back at the previous value. This indicates that the valves and rings are OK.
Poor Rings or Oil at Idle
The needle will be steady but the reading will be lower than normal.
Poor Rings or Oil when Throttle Snapped Open and Closed
Vacuum will drop to near zero and then momentarily climb to about 22 before settling back to previous value.
The needle will drop occasionally about 4 inches of mercury from its normal idling reading.
The needle will regularly drop by a greater amount than with a sticky valve – eg 6 inches of mercury.
The needle rapidly drops two or more inches each time the valve should be closed. May also indicate a misfire.
Loose Valve Guides
Fast vibration of the needle between 14 and 19 inches of mercury
Weak Valve Springs
When revving the engine hard, needle varies between 14 – 22 inches of mercury. As rpm increases, a wider variation occurs in needle movement.
Leaky Intake System
Needle reads low (eg below 5 inches of mercury) at idle. May also indicate late ignition timing or incorrect camshaft timing.
Leaky Head Gasket Between Cylinders
Needle floats regularly between 5 and 19 inches of mercury. May also indicate intake manifold gasket leak at an intake port or a faulty injector
High reading at first but suddenly returns to zero then builds slowly back to about 16 inches of mercury.
Clogged PCV or Intake Manifold Leaks
Needle moves slowly through a wide range of readings. Also check idle air/fuel ratio.
Obviously vacuum gauge readings cannot be definitive, but they can save a you a lot of time and money in chasing down obscure problems.