Engine Vacuum Test
To check manifold pressure with a vacuum gauge you need to locate a port on the intake manifold or throttle body. Manufacturer's install ports on their manifolds for lots of different reasons: Brake Booster, PCV tube, EGR Switch, A/C vents, etc. You simply need to find one small enough for the vacuum gauge line to slide onto firmly.
A quick snap of the throttle plate should drop the vacuum to around 5 "hg vacuum, and then it should recover to a steady 21 "hg vacuum. When this air is mixed with atomized fuel from the fuel injectors, engine speed is increased. For every 1000 ft. of increase in altitude, 1 "hg is subtracted from the vacuum reading.
Normal Engine: On a normal engine, accelerate to around 2000 rpm and then quickly release the throttle. The engine should snap right back to a steady 17- 21 "hg vacuum.
Steady low between 5-10 "hg vacuum: This indicates that the engine has a leak in the intake manifold or the intake gasket. This leak should be easy to find because it would be making a loud hissing noise.
Steady low between 10-15 "hg vacuum: This reading indicates late valve timing. There's a good chance the vehicle has jumped timing. Check the timing belt or chain depending on the application.
Steady low between 15-18 "hg vacuum: This just low reading indicates retarded ignition timing. Advance the timing on the distributor to correct this problem.
Fluctuating Needle: A fluctuating needle indicates there's a problem with a valve or a there's an engine misfire.
Needle drops during acceleration: If the needle drops steady during acceleration there's a restriction in the exhaust or intake. This is typically due to a clogged CAT or muffler.