Vacuum Gauge Diagnosis Chart - Engine Readings & What They Mean

This is a vacuum gauge diagnosis chart (© Craig Ueltzen, 1999) showing twelve illustrated gauge readings (0–30 in Hg scale) with color-coded needle behavior and the engine conditions each pattern indicates. A normal engine reads a steady 18"–20" at all speeds and briefly jumps 2"–5" above normal on quick deceleration. Abnormal readings and their diagnoses include: rapid wavering between 10"–20" (weak/broken valve spring); irregular wavering (intake port manifold leak); steady low reading under 2" below normal (retarded ignition timing); drifting at idle that stabilizes at higher RPM (burnt valve or combustion chamber leak); steady very low (late valve timing); wavering in one range regardless of engine speed (unbalanced carburetion, improper plug gap, ignition timing, poor valve seating); extremely low reading up to 15" drop (warped/cracked intake manifold or bad carburetor-to-manifold gasket); low average with ~3" fluctuation (worn valve guides); rhythmic 1"–7" drops (leaking chamber/valve or ignition failure in one cylinder) versus intermittent drops (sticking valve); needle hesitating above normal on deceleration (restricted exhaust/back pressure); drop to 0" under full throttle (dirty air cleaner, undersized carburetor, exhaust restriction); and little jump above normal when throttle is closed (worn piston rings causing blow-by). It is a general gasoline-engine diagnostic reference, not vehicle-specific.