A vacuum test can tell you a lot about an engine’s condition, similar to a cylinder leak down test. So, By doing a vacuum test in just 3-5 minutes you can know if an engine is healthy or not. A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. 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. You simply need to find one small enough for the vacuum gauge line to slide onto firmly. There are many factors that can affect the amount of vacuum created. Some of these are, Piston rings, Valves, Ignition system, Fuel control system and Emission Controls.