Intake vacuum are found in the intake manifold because of the pumping duties of the engine’s pistons and the restriction made by the throttle valve. But for the throttle choking off the flow of air into the engine, there would be little if any vacuum in the intake manifold (like in the case of a diesel). The bad side of intake vacuum is that it encourages pumping losses and reduces engine efficiency.

On older carbureted engines, vacuum is useful in pulling fuel into the engine. Vacuum siphons fuel through the idle, main metering and power circuits. An engine that has a vacuum leak, therefore, will likely be an engine that suffers from the signs of lean carburetion such as lean misfire, hesitation, stalling and rough idle. However the same symptoms can also be caused by a clogged catalytic converter or other exhaust restriction, a leaky EGR valve or valve timing problems (all of which reduce intake vacuum).

Engines with Multiport Fuel Injection and Gasoline Direct Injection don’t need vacuum to pull fuel into the engine because it is sprayed in under pressure. Even then, most of these engines still have a throttle for regulating airflow and the engine speed. And like the older carbureted engines, a throttle body also cause an airflow restriction that creates vacuum inside the intake manifold.

Generally, intake vacuum should be steady between 16 and 22 inches Hg (Mercury). A lower reading usually signals a vacuum leak, or any of the other problems just mentioned. A reading that gradually drops while the engine is idling almost often indicates an exhaust restriction. An oscillating vacuum reading usually indicates a leaky valve or badly worn valve guides that leak vacuum.

Although fuel injected engines do not depend on intake vacuum to pull fuel into the engine, vacuum leaks can upset the carefully balanced air/fuel ratio by allowing “unmeasured” air to enter the engine. The outcome is the same kind of drive ability signs as a vacuum leak on a carbureted engine (lean misfire, hesitation during acceleration, rough idle and even stalling). Common leak areas include injector O-rings, intake manifold gaskets, idle air control circuit and the throttle shaft.

Fuel injected engines also depend on intake vacuum to regulate the fuel pressure behind the injectors. Fuel delivery cannot be accurately measured until a fairly constant pressure differential is maintained. So the fuel pressure regulator diaphragm is connected to a source of intake vacuum. Vacuum working against a spring-loaded diaphragm inside the regulator opens a bypass that sends fuel back to the tank through a return line. This causes the fuel pressure in the injector rail to raise when engine load increases (and the vacuum drops). So, the regulator uses vacuum to maintain fuel pressure and the appropriate air/fuel ratio. A vacuum leak changes the equation by creating a drop in vacuum and a corresponding increase in line pressure.