Intake Manifold Vacuum Test   Burned or Warped Valves A regular, even spaced needle drop of about 4 inches or more. Can also be a sign of insufficient hydraulic lash adjuster clearance. Poor Valve Seating A small, but regular needle drop of about 1 or 2 inches. Worn Valve Guides When the needle swings back and forth over about a 4 inch range at idle, but will become steady as engine speed is increased. Weak Valve Springs When the needle swings back and forth over less then a 4 inch range during idle, but the range will increase as the engine speed is increased. Late Valve Timing A steady, but low reading. Ignition Timing Retardation A steady, but somewhat low reading. Insufficient Spark Plug Gap A small, regular pulse of the needle. Intake Leak A low steady reading. Blown Head Gasket A regular needle drop of a considerable amount. Restricted Exhaust System When the engine is started and at idle, the reading is normal, but as engine speed increases, the needle will slowly drop to 0 inches.