Gasoline Engine Vacuum Specifications and Testing (diagnosing)

Use the directions and read the safety warnings that came with your vacuum gauge.

    When testing, connect vacuum gauge to a vacuum port on intake manifold. You want the reading to come after the throttle plate. If you do not have to drive the vehicle, the brake booster vacuum hose is a good place to tap in. As you will get good volume. It is important not to drive the vehicle with a gauge tied into brake booster vacuum hose.

Engine Vacuum Specifications

  • Engine cranking, not starting: 2″ – 5″
  • Engine running at idle, warm: 18″ – 21″
  • Wide open throttle: 0″ – 3″
  • Engine deceleration: 21″ – 24″
  • Engine Vacuum – How to Test
    Snap throttle Jumps to 2″ then on decel moves to 25″ Healthy
    Snap throttle Jumps to 1″ then on decel moves to 22″ Fair but worn
    Idle 8″ or less Vacuum leak, loose intake manifold
    Idle Fluctuated between 14″ – 19″ Worn valve guides or head gasket blown between 2 cylinders
    Idle Reading drops from normal Burnt valve, valve stuck open, misfiring spark plug
    Idle 8″ – 14″ Valve timing incorrect or large camshaft overlap
    Idle 14″ – 16″ Ignition timing wrong
    Idle Moves between 12″ – 16″ Idle mixture incorrect
    Slow engine speed rise Needle falls then rises suddenly Blocked exhaustWeak valve springs
    3000 rpm Needle fluctuates and worsens with higher rpm Weak valve springs