Engine Vacuum Test and Compression Testing

Vacuum is produced by the engine's piston drawing air from the intake manifold during it's down stroke. The throttle plate keeps air from freely flowing into the manifold creating manifold vacuum. When the throttle body is opened there's a collapse or loss of vacuum. This collapse in vacuum is actually an increase in intake manifold air pressure. Vacuum is measured in inches of mercury (in. Hg).
A quick snap of the throttle plate should drop the vacuum to around 5 in. Hg (inches of mercury), and then it should recover to a steady 21 in. Hg. When this air is mixed with atomized fuel from the fuel injectors, engine speed is increased. It's important to remember for every 1000 ft. of increase in altitude 1 in. Hg is subtracted from the reading. For instance, if specifications required 21 in. Hg at an altitude of 3000 ft. above sea level, a reading of 18 in. Hg would be considered normal.
To check manifold pressure with a vacuum gauge you need to locate a port in the 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, and on and on. You simply need to find one small enough for the vacuum gauge line to slide onto firmly.

Perform a compression test to verify and test the integrity of the cylinders piston rings and valves. For instance, the engine in this illustration has 85 lbs. of compression in one of it's four cylinders. The key is the difference between the two readings of this cylinder after infecting oil. If the compression increases after adding the oil, it has increased the seal of the piston rings and resulted in higher readings. If the readings stay the same the valve or valve seat is faulty and the cylinder head would have to be removed and the valve/seat