Were it not for the throttle choking off the flow of air into the engine, there would be little if any vacuum in the intake manifold (like a diesel).
The downside of intake vacuum is that it creates pumping losses and reduces engine efficiency.
Vacuum siphons fuel through the idle, main metering and power circuits.
An engine with a vacuum leak, therefore, will likely be an engine that suffers from the symptoms of lean carburetion such as lean misfire, hesitation, stalling and rough idle.
But 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 so, most of these engines still have a throttle for regulating airflow and engine speed.
And like the older carbureted engines, a throttle body also creates an airflow restriction that creates vacuum inside the intake manifold.
On most engines, intake vacuum should be steady between 16 and 22 inches Hg (Mercury). A lower reading usually indicates a vacuum leak, or one of the other problems just mentioned.
A reading that gradually drops while the engine is idling almost always points to 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 rely on intake vacuum to pull fuelinto the engine, vacuum leaks can upset the carefully balanced air/fuel ratio by allowing "unmetered" air to enter the engine.
The result is the same kind of driveability symptoms as a vacuum leak on a carbureted engine (lean misfire, hesitation when accelerating, rough idle and possibly even stalling).
Common leak points include injector O-rings, intake manifold gaskets, idle air control circuit and the throttle shaft.
Fuel injected engines also rely on intake vacuum to regulate the fuel pressure behind the injectors.
Fuel delivery cannot be accurately metered unless 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 shunts fuel back to the tank through a return line. This causes the fuel pressure in the injector rail to rise when engine load increases (and vacuum drops).
Thus, the regulator uses vacuum to maintain fuel pressure and the correct air/fuel ratio.
A vacuum leak changes the equation by causing a drop in vacuum and a corresponding increase in line pressure.