Vacuum Gauge Diagnostic

Connect a vacuum gauge to a port on the intake manifold (NOT a ported source above the throttle plates). Readings are taken at idle on a warmed-up engine. Sea-level expectations; subtract roughly 1 in-Hg per 1,000 ft of elevation.

Needle Behavior	Reading	Likely Cause
Steady, strong	17–22 in-Hg	Healthy engine
Steady, low	10–15 in-Hg	Late ignition timing, intake/vacuum leak, or cam with lots of overlap
Steady, very low	5–10 in-Hg	Severely retarded timing, large vacuum leak, or stuck-open EGR
Slow drift across 3–5 in-Hg range	varies	Sticking valves; squirt penetrating oil into cylinders to confirm